A mammalian glutamic acid decarboxylase (GAD) cDNA probe has been utilized to isolate Drosophila cDNA clones that represent a genomic locus in chromosome region 64A. Deletion analysis indicates that this chromosomal locus encodes an enzymatically active GAD protein. The in vitro translation of cRNA representing a Drosophila cDNA clone yields a 57-kDa protein that can be immunoprecipitated by an anti-GAD antiserum. A GAD-immunoreactive protein of the same size can also be detected in Drosophila head extracts. The nucleotide sequence derived from two overlapping Drosophila cDNA clones predicts a 57,759-dalton protein composed of 510 residues that is 53% identical to mammalian GAD. Sequence comparisons of mammalian and Drosophila GAD identify two highly conserved regions (greater than or equal to 70% identity), one of which encompasses a putative co-factor-binding domain. Transcriptional analyses show that expression of the Drosophila Gad gene commences early in embryonic development (4-8 h) and continues in all later developmental stages. A 3.1-kb class of mRNA is detected throughout embryogenesis, in all three larval stages, in pupae, and in adults. This transcript class has a widespread distribution in the adult CNS. A smaller 2.6-kb transcript is expressed in a developmentally regulated manner; it is detected only in embryos and pupae.