TheDrosophilareceptor oscillation A(rosA)mutations, which cause electroretinogram (ERG) defects, including oscillations, were localized to the 24F4-25A2 region of chromosome 2L. Genomic fragments from this region, isolated from bacteriophage PI clones, included those that detect transcriptional defects inrosAmutants in RNA blot experiments. One of these genomic fragments was used to screen a head cDNA library. The largest cDNA clone (3.6 kb) isolated was shown to rescue arosAmutant in P element-germline transformation experiments. The ROSA protein deduced from the open reading frame in the 3.6 kbrosAcDNA is 943 amino acids long and is 36-41% identical to members of the superfamily of Na+/Cl−-dependent neurotransmitter transporters, with no indication of higher sequence identity to any one subgroup within the superfamily. RNA blot experiments revealed multiple transcripts in various developmental stages, the most abundant one being a 3.7 kb transcript, particularly in the adult head. Tissuein situexperiments identified therosAtranscript to be localized to many tissues, with higher levels of hybridization in the nervous system and digestive tract. The results demonstrate that therosAgene encodes a novel Na+/Cl−-dependent transporter important for normal response properties of the photoreceptor.