Sex ratio distortion induced by a male-killing agent has been found to affect Drosophila melanogaster. The trait was discovered accidentally in a collection of flies from markets in Campinas, São Paulo State, Brazil. Repeated crosses with Canton-S males (for 15 generations to date) and successful transmission using the injection of macerates of sex ratio flies, have shown that the trait is inherited maternally, is cytoplasmic and is infectious. Crosses with strains marked with the visible mutation white and viability experiments at pre-adult stages of development, indicate that the skewed sex ratio results from male mortality before hatching. Males do not transmit the trait to their progeny.