This paper describes a fast and efficient approach to correlating cloned genes with mutant phenotypes in Drosophila. We make use of a large collection D. melanogaster lines with recessive lethal insertions of a P[lacW] transposon on their second chromosome. Within this collection there clearly must be many insertions corresponding to Drosophila genes that have been cloned and characterized, e.g., via homology with cloned mammalian genes, but for which mutant phenotypes have yet to be identified. We have made use of the fact that P[lacW] contains a plasmid replicon to establish a collection of rescued plasmids containing genomic DNA flanking the sites of transposon insertion. Plasmids representing a total of 1836 lines were independently rescued and pooled in batches of 10 and 100. Pools of 100 plasmids were screened by hybridization with cDNAs corresponding to cloned second chromosome loci. Hybridizing pools were then narrowed down to single plasmids by a process of subdivision and rehybridization, and corresponding mutant lines were obtained. The success rate was better than one in four. This rate would undoubtedly be improved by the use of genomic DNA probes.