Organisms are constantly being bombarded by stimuli in their envi­ ronment (and also by internal stimuli), and a common way of responding is by movement. This is an aspect of irritability, or excitability, or behaviour. Response to stimuli by movement is found in all organisms: it represents one of the universalities of biology. Yet at the molecular level it is one of the least understood of biological phenomena. Micro-organisms are no exception. If motile, they respond to stimuli by active movement (taxis); if sessile, they respond by growth movements (tropisms). Responses by movement are known among micro-organisms to such stimuli as chemicals, electric current, gravity, light, temperature, touch, and vibrations. The behaviour of micro-organisms is an exciting subject, first of all for its own sake, but in addition because it may reveal facts and concepts that are applicable to understanding behaviour in more complicated organisms (even us) and because it may, help to understand the movement of cells and tissues during differentiation and development of higher plants and animals.