In the genome of Drosophila melanogaster there is only one locus, Tpl, that is triplo-lethal; it is also haplo-lethal. Previous work has identified 3 hypomorphic alleles of Tpl which rescue animals carrying a duplication of Tpl, but which are not dominant lethals as null mutations or deficiencies would be. We have found that all three hypomorphic alleles act as site-specific hotspots for recombination when heterozygous with a wild-type homolog. Recombination between the flanking markers ri and Ki is increased 6.5-10.5-fold in the presence of Tpl hypomorphic alleles. The increased recombination was found to occur between Tpl and Ki, while recombination in other adjacent regions is unchanged. The use of isogenic Tpl+ controls, and the use of flanking intervals in the mutant chromosomes allows us to rule out the interchromosomal effect as a cause. We have also observed premeiotic recombination occurring at the Tpl hypomorphic alleles in male heterozygotes. We hypothesize that transposons are responsible for both the hypomorphic phenotype and the high frequency of recombination.