This paper describes a method for recognizing the environment and selecting behaviors for a humanoid robot that are appropriate to the environment, based on sensor histories. In order for humanoid robots to move around many kinds of environments, one of the keys is selecting the appropriate behavior. Because features of an environment that deeply influence a robot's motion, such as viscous friction or floor elasticity, are difficult to immediately measure before moving, it is difficult for the robots to plan their behaviors to take these factors into account. Therefore, in our method, a robot first records a long time series of data from sensors attached to its body while performing each behavior in each environment, and it builds decision trees based on this data for each of the behaviors, enabling it to recognize the current environment based on the sensor history. By using the trees, the robot selects the behavior which will be most effective for recognizing the environment, acquires a sensor history while doing the behavior, and selects likely candidates for the environment. By iterating over these steps, the robot can recognize the environment and select an appropriate behavior. To verify the validity of the method, we used a small-size humanoid robot and conducted an experiment to recognize environments in a family's house. © 2006 IEEE.