The nucleoside analogue N4-aminocytidine is known to induce mutations in bacteria, and was shown to induce somatic mutations in Drosophila melanogaster after larval administration. The assay system employed was a wing-hair mutation spot test developed by Würgler and co-workers. The potency of N4-aminocytidine to induce somatic mutation is comparable to those of several food-pyrolysate mutagens previously reported. The occurrence of twin spots, i.e. two types of recessive mutant-hair clones in adjacent positions, suggests that N4-aminocytidine induces somatic recombination in Drosophila. Another feature of the mutagenicity of N4-aminocytidine is that both the acute and the chronic larval feedings gave rise to mutant hair formation of similar patterns with respect to the spot-size distributions: small single spots were formed predominantly and the larger the spot-size, the lower their frequency.