1. The movements of Tubularia spermatozoa in the vicinity of micropipettes filled with extracts of female hydranths, which chemotactically attract the spermatozoa, have been recorded by multiple-flash photomicrography.
2. When a spermatozoon turns in response to a chemotactic stimulus, the flagellum continues to beat, with a highly asymmetrical pattern of bending, during the turn.
3. The magnitude of the turn, particularly the duration of the period of asymmetrical beating, is variable, but each spermatozoon is only able to make turns in one direction, relative to its own body.
4. Most of the behaviour of these spermatozoa may be explained if the turning mechanism is activated when the spermatozoon detects a decreasing concentration of the chemotactant
