Therpa(receptor potential absent) mutation of the blowfly,Calliphora erythrocephala, reduces the light-evoked responses of photoreceptor cells and renders the fly blind. This phenotype is similar to the phenotype caused bynorpAmutations inDrosophilawhich have been shown to occur within a gene encoding phospholipase C. In Western blots,norpAantiserum stains a protein in homogenates of wild-typeCalliphoraeye and head that is similar in molecular weight to thenorpAprotein. Very little staining of this protein is observed in similar homogenates ofrpamutant. Moreover,norpAantiserum strongly stains retina in immunohistochemical assays of wild-type adult head, but not inrpamutant. Furthermore, eyes ofrpamutant have a reduced amount of phospholipase C activity compared to eye of wild-typeCalliphora. These data suggest that therpamutation occurs in a phospholipase C gene of the blowfly that is homologous to thenorpAgene ofDrosophila.