The Drosophila mutant no receptor potential A (norpA) is the phototransduction-defective mutant which lacks phosphatidylinositol-specific phospholipase C (PI-PLC) activity. Recently, norpA cDNA was isolated and its homology to a bovine PI-PLC was demonstrated [Bloomquist, B.T. et al. (1988) Cell 54, 723–733]. On the basis of its cDNA, we determined the genomic organization of the norpA gene and revealed that the norpA gene consists of 13 coding exons spreading over a 15 kb genomic area. Furthermore, we identified the mutational sites of two temperature-sensitive (ts) mutants. The analysis of norpAHB2 revealed that the single amino acid change from Ser-551 to Tyr causes the PI-PLC activity to become temperature-sensitive. The other allele, norpAKO50, has two substitutions from Ser-406 to Phe and from Gly-451 to Ser. These three mutations are located in regions highly conserved in other mammalian PI-PLC molecules. This suggests that these regions are important for PI-PLC catalytic activity.