There is ample epidemiological evidence showing that sunlight can cause skin cancer in the human. In experimental studies, simulated sunlight or UV lamps are used for demonstrating carcinogenesis and other biological effects. Little studies, however, have been performed using natural sunlight itself. In this work, we have examined the mutagenicity of natural sunlight in Drosophila. The Drosophila wing spot test is useful to detect somatic cell mutations. Third instar larvae in petri dishes were exposed to sunlight (ultraviolet region with <290 nm wavelength cut off by a plastic cover) in the yard of Okayama University campus (north latitude: 34* 39', east longitude: 133?55'). The sunlight was mutagenic in Drosophila larvae and produced pyrimidine dimers in their DNA. In the observed mutagenicity, there was dependence on the exposure period and UV fluence. During the two-year monitoring, the highest induction of mutant spot observed was 1.98 total spots/wing on June 25, 1998, and the lowest was 0.64 on December 29, 1998, while non-exposure spontaneous spots were 0.29 and 0.32 on these days, respectively. Thus, solar radiation was mutagenic both in summer and in winter. J Epidemiol, 1999 ; 9 : S66-S71.