THODAY and Boam1–3 have shown that disruptive selection can render a population of Drosophila melanogaster effectively polymorphic for factors affecting sternopleural chaeta number. That such selection might produce polymorphism was predicted by Mather4. Thoday and Boam3 found that the polymorphism they had established depended upon a second-chromosome giving low chaeta numbers and a third-chromosome giving high chaeta numbers, each of these being balanced against chromosomes giving intermediate chaeta numbers. We have carried out further experimental analysis of second-chromosomes extracted from these polymorphic populations with remarkable results which, though there is much further work to do, seem to us worth immediate report.