A small, divergently transcribed gene is located 500 bp upstream of the suppressor of Hairy-wing locus of Drosophila melanogaster. Sequencing of a full-length cDNA clone of the predominant 850-nucleotide transcript reveals that this gene encodes a 15,100-Da protein with high homology to a subunit of RNA polymerase II. The RpII15 protein is 46% identical to the RPB9 protein of Saccharomyces cerevisiae, one of the smallest subunits of RNA polymerase II from that species. Among those identical residues are four pairs of cysteines whose spacing is suggestive of two metal-binding "finger" domains. The gene is expressed at all developmental stages and in all tissues. Two deletions within the RpII15 gene are multiphasic lethal deletions, with accumulation of dead animals commencing at the second larval instar. Ovary transplantation experiments indicate that survival of mutant animals to this stage is due to the persistence of maternal gene product throughout embryogenesis and early larval development. The RpII15 gene product is thus necessary for viability of D. melanogaster.