Phosphatidylinositol phosphodiesterase activity was found to be entirely absent in the head of the Drosophila visual mutant, norpA. It was also demonstrated that the enzyme activity was highly concentrated in the retinular cells of a normal head. Furthermore, the enzyme was revealed to be in a membrane bound form. In view of the present results and our previous work on the reduction of diacylglycerol kinase activity in the norpA mutant, phosphatidylinositol metabolism may play an important role in the generation of photoreceptor potentials.