ONE of the more interesting behavioural characteristics of the cellular slime mould, Dictyostelium discoideum, involves the concerted migration of thousands of amoebae in a pseudoplasmodium or slug towards light. There have been several studies of the phenomenon, and the action spectrum for phototaxis has been shown1–4 to have peaks at 425 and 550 nm. The biochemical process by which light energy controls the direction of movement, however, remains unknown. Moreover, the mechanism of locomotion which can carry a 1 mm long slug for several centimetres in a period of 2–3 days is unclear, but seems to involve more than directed amoeboid motion of the individual cells. Because the amoebae in a slug move through a casing of slime secreted as a viscous fluid by the tip cells5, it seems probable that both the extent and the direction of migration would be dependent on the rate of formation of slime casing. Because further biochemical analysis might benefit from the availability of mutants which lack the phototactic response, conditions which would make it possible to select such mutant strains were developed.