Developing cells of the cellular slime mold Dictyostelium discoideum release a low molecular weight metabolite (Slug Turning Factor, STF) which, at high uniform concentrations, interferes with phototaxis and thermotaxis by D. discoideum slugs. D. discoideum slugs migrating in darkness are repelled by (exhibit negative chemotaxis to) crude STF exudates. Dose-response curves relating the accuracies of phototaxis and negative chemotaxis to STF concentration indicate that, in both phototaxis and chemotaxis, slugs "measure" the ratios of STF concentrations on their opposite sides. Net STF release is enhanced by light. We propose that light, focused onto the slug's distal side by its convex surface, generates a lateral STF gradient in response to which the slug turns toward the light source.