The transient receptor potential (trp) mutant in Drosophila is known to manifest retinal degeneration and involves defects in the intermediate steps of visual transduction. The chromosome walking technique has been conducted at the cytogenetic location of trp. Overlapping phage clones that cover a DNA stretch of 73 kb were obtained. Southern blotting studies detected a 2.3 kb DNA deletion within this region in the mutant. The deleted DNA fragment contains exons of three transcripts with lengths of 3.5, 1.5 and 0.8 kb. The transcription of the 3.5 and 1.5 kb RNAs is completely abolished in the mutant, whereas that of 0.8 kb RNA appears to be modified. The impairment in production of three RNAs, due to the DNA deletion, offers a possible molecular basis of the multiple defects displayed by the trp- phenotype.