Dictyostelium slug phototaxis and thermotaxis are readily assayed phenotypes that reflect with great sensitivity and specificity the interactions of environmental stimuli with morphogenetic signaling systems controlling the collective movement of slug cells. Methods are described for conducting and recording phototaxis, thermotaxis, and spontaneous turning experiments, although it is pointed out that spontaneous turning rates are not easily measured in most strains of molecular biological interest. Both phototaxis and thermotaxis can be assayed qualitatively for rapid screening of prospective mutants and quantitatively for detailed phenotypic analysis. Both types of assay are simple to conduct, but require care to avoid the potentially misleading effects of other factors such as cell density and extraneous thermal and chemical gradients that might influence slug behavior. The quantitative analysis and statistical testing of conclusions are carried out using directional statistics, because traditional statistical methods for linear data are inappropriate and potentially misleading when applied to directional data. The appropriate statistical methods are given for measuring maximum likelihood estimates and confidence intervals for the average direction (mu) and the accuracy of orientation (kappa) for unidirectional orientation, as well as the two preferred directions (+/-alpha) and accuracy of orientation in bidirectional phototaxis. In addition, tests for uniformity (kappa = 0), for equality of kappa in the two-sample and multisample cases, and for bidirectional phototaxis vs unidirectional phototaxis are described. These methods are readily implemented in the R environment for statistical computing and the functions required to do so are described and provided.