Dictyostelium slugs are able to respond to environmental stimuli in an extremely sensitive and efficient way. This enables a slug to migrate to more favourable locations for formation of fruiting bodies and dispersal of spores. Phototaxis is a readily assayed phenotype and reflects the interactions of environmental stimuli with morphogenetic signalling systems controlling the movement of the slug. The methods for assaying phototaxis are described here. Qualitative phototaxis tests are described and can be used for rapid screening of potential mutants or effects of pharmacological agents. These tests are simple to conduct yet care must be taken in order to avoid the effects of high cell density which can be misleading when interpreting results. Quantitative phototaxis tests can be performed with known cell densities of amoebae which ensures that any effects seen are caused by the mutation or pharmacological agent and not simply due to differences in cell densities.