Phototransduction is the process by which light-stimulated photoreceptor cells of the visual system send electrical signals to the nervous system. Many of the steps that follow the initial event in phototransduction, absorption of light by rhodopsin, are ill-defined. The fruitfly, Drosophila melanogaster, provides a means to dissect phototransduction genetically. Mutations such as transient receptor potential (trp) affect intermediate steps in phototransduction. In order to facilitate molecular studies of phototransduction, the trp gene was isolated and its identity was confirmed by complementing the mutant trpCM allele of the trp gene by P-element mediated germline transformation of a 7.1-kilobase DNA fragment. Expression of the trp gene begins late in pupal development and appears to be limited to the eyes and ocelli.