We have used DNA-mediated gene transfer to introduce into Drosophila melanogaster cells DNA sequences for which no selective criteria exist. We have introduced a Drosophila heat-shock locus into cultured Drosophila cells by calcium phosphate cotransfection with the copia vector pCV31gpt and selection for xanthine utilization. We recovered cell lines containing between three and about 50 copies of both pCV31gpt and pMH10A, a cloned 87 A7 hsp70 heat-shock locus that encodes a mutant 40,000-dalton heat-shock protein (hsp40). The stable inheritance of the transformed DNAs argues that the input DNAs have integrated into the genome. We show that this is indeed the case for one cell line by cloning back the transfected DNA and detecting the flanking chromocentral sequences by in situ hybridization. Surprisingly, the integrated hsp70 genes are not expressed. This report represents the first example of the cointroduction of DNA sequences into Drosophila cells by cotransfection with a dominant selectable marker.