Larvae of Drosophila melanogaster were reared on a medium to which 0.8% of pyronin-B had been added, and recessive sex-linked and autosomal lethals were scored in successive broods from individual treated males. It was found that the mutagenic action of pyronin-food, unlike that of formaldehyde-food, extends backwards into young spermatogonial stages. Heterogeneity between treated animals is very marked and results in a low overall mutation rate. There is a suggestion that oogonia, which are refractory to formaldehyde, respond to pyronin. Apparently, the mechanism of mutagenesis is different for these two types of food.