The cGMP-dependent protein kinase (PKG) has many cellular functions in vertebrates and insects that affect complex behaviors such as locomotion and foraging. The "foraging" ("for") gene encodes a PKG in "Drosophila melanogaster." Here, we demonstrate a function for the "for" gene in sensory responsiveness and nonassociative learning. Larvae of the natural variant sitter ("for[superscript s]") show less locomotor activity during feeding and have a lower PKG activity than rover ("for[superscript R]") larvae. We used rover and sitter adult flies to test whether PKG activity affects (1) responsiveness to sucrose stimuli applied to the front tarsi, and (2) habituation of proboscis extension after repeated sucrose stimulation. To determine whether the differences observed resulted from variation in the "for" gene, we also tested "for[superscript s]", a sitter mutant produced on a rover genetic background. We found that rovers ("for[superscript R]") were more responsive to sucrose than sitters ("for[superscript s]" and "for[superscript s2])" at 1-, 2-, and 3-wk old. This was true for both sexes. Differences in sucrose responsiveness between rovers and sitters were greater after 2 h of food deprivation than after 24 h. Of flies with similar sucrose responsiveness, "for[superscript R]" rovers showed less habituation and generalization of habituation than "for[superscript s]" and "for[superscript s2]" sitters. These results show that the PKG encoded by "for" independently affects sensory responsiveness and habituation in "Drosophila melanogaster." (Contains 1 table and 7 figures.)