The quemao (qm) locus of Drosophila melanogaster is characterized by a P-element-associated mutant lacking most of the large bristles on the thorax and by several EMS-induced recessive lethals. quemao was cloned using a transposon tagging strategy. P-element-mediated transformation demonstrated that the cloned qm DNA sequence (from the 65F cytological region) rescues the mutant phenotype. A 2.3-kb qm transcript was identified by Northern blot analysis by sequencing of the isolated qm cDNA clones and by 5′ rapid amplification cDNA end (RACE). The predicted amino acid sequence (338 residues) of the coding region of the qm transcript shares 42, 31, 13, 20, and 12% identical amino acid sequences with the geranylgeranyl pyrophosphate synthase (GGPPS) of fungi, yeast, plants, archaebacteria, and eubacteria, respectively. It also contains five highly conserved domains common among all known isoprenyl pyrophosphate synthases. The P element associated with the original qm mutant is inserted in the 5′ untranslated region of the transcript. An EMS-induced qm nonsense mutation at the 12th codon leads to recessive lethality at the first larval instar, indicating the essential role of qm in the isoprenoid biosynthesis of insects.