ABSTRACT Non-Mendelian inheritance was revealed for the "heat-sensitivity" character of the poikilothermic insect Drosophila melanogaster. Genetic analyses were performed on heat-sensitive (S, S 1) strains, derived through indirect selection, and on stocks constructed through extensive chromosomal and cytoplasmic substitutions between strains obtained from two replicate cage populations. The populations were kept for about 7 years under different temperatures (14°-25°) and exhibited different survival. We conclude that the character studied is quantitative, responds to selection pressure and is transmitted through the maternal cytoplasm, while nuclear genes modify its expression.