Weak and strong light/heat stimuli induced changes in cyclic guanosine monophosphate (cGMP) levels in vegetative and aggregation competent amoebae and in slug cells of the Dictyostelium discoideum strain X22. Mutant strains derived from X22 with mutations in the phototaxis loci phoA-phoK fell into four phenotypic classes with respect to cGMP responses to weak and strong light/heat stimuli. These results suggest an intermediary role for cGMP in photosensory and thermosensory processing in slugs and amoebae. The streamer F mutant NP368 which has previously been shown to exhibit a prolonged cGMP response to cAMP, showed a wild-type cGMP response to light and heat. All phototaxis mutant strains with altered cGMP responses to light and heat were unaltered in their cGMP response to cAMP. These results suggest that cAMP and light/heat regulate cGMP via independent pathways. The thermotaxis mutant HPF228 showed altered cGMP responses to heat but not to light stimuli. This suggests that the mutation in HPF228 affects thermosensory transduction before convergence with the phototaxis pathway and the subsequent cGMP response.