The tko (technical knockout) mutation is one of a family of behavioral mutations that cause "bang sensitivity" in D. melanogaster. Using P-element-mediated transformation, we show that a 3.1 kb piece of genomic DNA complements tko. This fragment contains only one complete transcript, 0.68 kb in length. This transcript is abundantly expressed through all stages of the life cycle, and we have isolated cDNAs corresponding to this transcript. Their sequence implies a protein product composed of 140 amino acids, which exhibits considerable sequence similarity to ribosomal protein S12 from both Euglena gracilis chloroplasts and E. coli. We suggest that tko codes for a mitochondrial ribosomal protein and that the tko phenotype results from defective mitochondria.