A semi-lethal, sterile allele of the smooth locus (2-91.5), sm3, was discovered in an artificial selection line for low abdominal bristle number that had been started from a P-M dysgenic cross. The fitness effects and extremely low bristle number phenotype of the allele could not be separated by recombination from a P element insertion at cytological location 56E, and precise excision of the P element at this site was associated with reversion to wild type. The smooth gene was cloned using the P element insertion as a tag. The gene encodes a 2.6 kb transcript derived from 10 exons, and covers a genomic region of at least 74 kb. The Drosophila smooth gene is homologous to a group of RNA binding proteins, with the strongest homology to the human heterogeneous nuclear ribonucleoprotein L gene. The smooth gene is by definition an abdominal bristle number quantitative trait locus, but further work is required to discern whether naturally occurring allelic variation at this locus is a source of genetic variation for abdominal bristle number in natural populations.