The Drosophila gamma-aminobutyric acid (GABA) receptor subunit gene Rdl was isolated on the basis of a mutant phenotype showing high levels of insensitivity to picrotoxinin and cyclodiene insecticides. Following analysis of two dissimilar cDNAs isolated from the locus, we report that Rdl undergoes extensive alternative splicing at two locations in the putative extracellular domain. At each location a choice is made between exons of the same size: "a" or "b" (23 amino acids long with two substitutions) and "c" or "d" (46 residues long with 10 substitutions). The function of these alternative exons remains unclear; however, exon d contains a putative site for casein kinase II phosphorylation. All possible combinations of exons (a with c or d and b with c or d) were found in RNA isolated from early embryos. This is the first demonstration of alternative splicing in a GABA receptor gene from invertebrates.