Two classes of phosphorylated homologs of vertebrate arrestins, designated phosrestins I (PRI) and phosrestin II (PRII), are expressed in the photoreceptors of a fruit fly, Drosophila melanogaster. This study presents evidence that the housefly, Musca domestica, also has a protein similar to Drosophila PRI. Our conclusion is based on the following evidence. (1) We identified a Musca photoreceptor protein exhibiting a molecular mass (51 kDa) and an isoelectric point (pI = 8.6) similar to those of Drosophila PRI. This Musca protein, designated Musca PRI, changes its pI upon illumination in vivo. Drosophila PRI. This Musca protein, designated Musca PRI, changes its pI upon illumination in vivo. (2) Rabbit antibodies raised against Musca PRI, against bovine arrestin, and against a synthetic peptide based on the Drosophila PRI sequence stained the Drosophila and Musca PRIs specifically on 1 and 2-dimensional Western immunoblots. (3) Both Drosophila and Musca PRIs incorporated 32P-radioactivity from gamma-32P-ATP in cell-free homogenates of retinas. Partial peptide digestions of Drosophila and Musca PRIs revealed similarity between these proteins. We observed that Drosophila PRI exists in the random preparation, but it also exists in other subcellular fractions. Immunocytochemistry at the EM level revealed a distribution of both Drosophila and Musca PRI epitopes in membranous vesicular structures in the cytosol as well as in the rhabdomeric microvillar membranes where the visual pigment, rhodopsin, exists. Such distribution of PRI epitopes suggests that PRI and its light-dependent phosphorylation may function in a space remote from the rhabdomere as well as the immediate milieu of photoreception.