diff --git a/whynot/gym/__init__.py b/whynot/gym/__init__.py
index 30c32f8..9c9f0aa 100644
--- a/whynot/gym/__init__.py
+++ b/whynot/gym/__init__.py
@@ -5,10 +5,8 @@
 import sys
 import warnings
 
-from gym import error
-from gym.core import Env
-from gym import logger
+from gymnasium import error
+from gymnasium.core import Env
+from gymnasium import logger
 
-from whynot.gym.envs import make, spec, register
-
-__all__ = ["Env", "make", "spec", "register"]
+__all__ = ["Env"]
diff --git a/whynot/gym/envs/__init__.py b/whynot/gym/envs/__init__.py
index 176ce29..da71253 100644
--- a/whynot/gym/envs/__init__.py
+++ b/whynot/gym/envs/__init__.py
@@ -1,4 +1,3 @@
 """Environments based on WhyNot simulators."""
 
-from whynot.gym.envs.registration import registry, register, make, spec
 from whynot.gym.envs.ode_env import ODEEnvBuilder
diff --git a/whynot/gym/envs/registration.py b/whynot/gym/envs/registration.py
deleted file mode 100644
index 60613a0..0000000
--- a/whynot/gym/envs/registration.py
+++ /dev/null
@@ -1,28 +0,0 @@
-"""Global registry of environments, for consistency with openai gym."""
-import importlib
-
-from gym.envs.registration import EnvRegistry
-
-# Keep for consistency with original API
-# pylint:disable-msg=invalid-name
-# Have a global registry
-registry = EnvRegistry()
-
-
-# pylint:disable-msg=redefined-builtin
-def register(id, **kwargs):
-    """Register the environment."""
-    return registry.register(id, **kwargs)
-
-
-def make(id, **kwargs):
-    """Build the environment."""
-    return registry.make(id, **kwargs)
-
-
-def spec(id):
-    """View the spec for the environment."""
-    return registry.spec(id)
-
-
-warn_once = True
diff --git a/whynot/gym/spaces/__init__.py b/whynot/gym/spaces/__init__.py
index 31dbfd3..3bf2a06 100644
--- a/whynot/gym/spaces/__init__.py
+++ b/whynot/gym/spaces/__init__.py
@@ -1,15 +1,15 @@
-"""Ensure gym spaces are accessible if you import whynot.gym as gym."""
-from gym.spaces.space import Space
-from gym.spaces.box import Box
-from gym.spaces.discrete import Discrete
-from gym.spaces.multi_discrete import MultiDiscrete
-from gym.spaces.multi_binary import MultiBinary
-from gym.spaces.tuple import Tuple
-from gym.spaces.dict import Dict
+"""Ensure gymnasium spaces are accessible if you import whynot.gym as gym."""
+from gymnasium.spaces.space import Space
+from gymnasium.spaces.box import Box
+from gymnasium.spaces.discrete import Discrete
+from gymnasium.spaces.multi_discrete import MultiDiscrete
+from gymnasium.spaces.multi_binary import MultiBinary
+from gymnasium.spaces.tuple import Tuple
+from gymnasium.spaces.dict import Dict
 
-from gym.spaces.utils import flatdim
-from gym.spaces.utils import flatten
-from gym.spaces.utils import unflatten
+from gymnasium.spaces.utils import flatdim
+from gymnasium.spaces.utils import flatten
+from gymnasium.spaces.utils import unflatten
 
 __all__ = [
     "Space",
diff --git a/whynot/gym/utils/__init__.py b/whynot/gym/utils/__init__.py
index 66ab481..be867da 100644
--- a/whynot/gym/utils/__init__.py
+++ b/whynot/gym/utils/__init__.py
@@ -1,2 +1,2 @@
 """Ensure gym utils are accessible if you import whynot.gym as gym."""
-from gym.utils import seeding
+from gymnasium.utils import seeding
diff --git a/whynot/simulators/credit/environments.py b/whynot/simulators/credit/environments.py
index 02dab5b..3a448d9 100644
--- a/whynot/simulators/credit/environments.py
+++ b/whynot/simulators/credit/environments.py
@@ -1,10 +1,11 @@
 """Interactive environment for the credit simulator."""
 import copy
+import gymnasium
 
 import numpy as np
 
 from whynot.gym import spaces
-from whynot.gym.envs import ODEEnvBuilder, register
+from whynot.gym.envs import ODEEnvBuilder
 from whynot.simulators.credit import (
     Config,
     Intervention,
@@ -93,9 +94,10 @@ def build_credit_env(config=None, initial_state=None):
     )
 
 
-register(
+gymnasium.register(
     id="Credit-v0",
     entry_point=build_credit_env,
+    apply_api_compatibility=True,
     max_episode_steps=100,
     reward_threshold=0,
 )
diff --git a/whynot/simulators/hiv/environments.py b/whynot/simulators/hiv/environments.py
index 2309cdd..89f7918 100644
--- a/whynot/simulators/hiv/environments.py
+++ b/whynot/simulators/hiv/environments.py
@@ -6,10 +6,11 @@
 
 https://pdfs.semanticscholar.org/c030/127238b1dbad2263fba6b64b5dec7c3ffa20.pdf
 """
+import gymnasium
 import numpy as np
 
 from whynot.gym import spaces
-from whynot.gym.envs import ODEEnvBuilder, register
+from whynot.gym.envs import ODEEnvBuilder
 from whynot.simulators.hiv import Config, Intervention, simulate, State
 
 
@@ -59,6 +60,8 @@ def observation_space():
     reward_fn=get_reward,
 )
 
-register(
-    id="HIV-v0", entry_point=HivEnv, max_episode_steps=400, reward_threshold=1e10,
+gymnasium.register(
+    id="HIV-v0", entry_point=HivEnv, apply_api_compatibility=True, max_episode_steps=400, reward_threshold=1e10,
 )
+
+env = gymnasium.make("HIV-v0", apply_api_compatibility=True)
diff --git a/whynot/simulators/opioid/environments.py b/whynot/simulators/opioid/environments.py
index 1427e00..932234b 100644
--- a/whynot/simulators/opioid/environments.py
+++ b/whynot/simulators/opioid/environments.py
@@ -1,8 +1,10 @@
 """Reinforcement learning environments for the opioid epidemic simulator."""
+import gymnasium
+
 import numpy as np
 
 from whynot.gym import spaces
-from whynot.gym.envs import ODEEnvBuilder, register
+from whynot.gym.envs import ODEEnvBuilder
 from whynot.simulators.opioid import Config, Intervention, simulate, State
 
 
@@ -58,9 +60,10 @@ def observation_space():
     reward_fn=get_reward,
 )
 
-register(
+gymnasium.register(
     id="opioid-v0",
     entry_point=OpioidEnv,
+    apply_api_compatibility=True,
     # The simulator starts in 2002 and ends in 2030.
     max_episode_steps=28,
     reward_threshold=0,
diff --git a/whynot/simulators/world3/environments.py b/whynot/simulators/world3/environments.py
index 32adf7f..423fdd5 100644
--- a/whynot/simulators/world3/environments.py
+++ b/whynot/simulators/world3/environments.py
@@ -1,10 +1,12 @@
 """Reinforcment learning for world3."""
+import gymnasium
+
 from itertools import product
 
 import numpy as np
 
 from whynot.gym import spaces
-from whynot.gym.envs import ODEEnvBuilder, register
+from whynot.gym.envs import ODEEnvBuilder
 from whynot.simulators.world3 import Config, Intervention, simulate, State
 
 
@@ -63,6 +65,6 @@ def observation_space():
     reward_fn=get_reward,
 )
 
-register(
-    id="world3-v0", entry_point=World3Env, max_episode_steps=400, reward_threshold=1e5,
+gymnasium.register(
+    id="world3-v0", entry_point=World3Env, apply_api_compatibility=True, max_episode_steps=400, reward_threshold=1e5,
 )
diff --git a/whynot/simulators/zika/environments.py b/whynot/simulators/zika/environments.py
index ae68b62..8212da6 100644
--- a/whynot/simulators/zika/environments.py
+++ b/whynot/simulators/zika/environments.py
@@ -6,10 +6,12 @@
 
 https://www.sciencedirect.com/science/article/pii/S2211692316301084#!
 """
+import gymnasium
+
 import numpy as np
 
 from whynot.gym import spaces
-from whynot.gym.envs import ODEEnvBuilder, register
+from whynot.gym.envs import ODEEnvBuilder
 from whynot.simulators.zika import Config, Intervention, simulate, State
 
 
@@ -78,6 +80,6 @@ def action_space():
     reward_fn=get_reward,
 )
 
-register(
-    id="Zika-v0", entry_point=ZikaEnv, max_episode_steps=200, reward_threshold=1e10,
+gymnasium.register(
+    id="Zika-v0", entry_point=ZikaEnv, apply_api_compatibility=True, max_episode_steps=200, reward_threshold=1e10,
 )