The movement of a cell (e.g. a gamete) or a microorganism in response to an external stimulus. Certain microorganisms have a light-sensitive region that enables them to move towards or away from high light intensities (positive and negative phototaxis respectively). Many bacteria move in response to chemical stimuli (chemotaxis); a specific example is aerotaxis, in which atmospheric oxygen is the stimulus. Taxic responses are restricted to cells that possess cilia, flagella, or some other means of locomotion. The term is usually not applied to the movements of higher animals. See also geotaxis. Compare kinesis; tropism.