To monitor genotoxicity in small bodies of water ples from agricultural areas and the Bruce Penin-(e.g., creeks, ponds, and drainage ditches) we ex-sula. R. clamitans tadpoles showed significant anamined tadpole erythrocytes of two species: Rana nual variation in DNA damage which was greater clamitans and Rana pipiens, using the alkaline sin-in samples of tadpoles collected from agricultural gle cell gel DNA electrophoresis (SCG) or ''comet'' areas than from the Bruce Peninsula. The higher assay. This approach involves detection, under al-levels of DNA damage in tadpoles collected from kaline conditions, of cell DNA fragments which on agricultural areas may be due to the pesticides electrophoresis migrate from the nuclear core, re-used, and the increased variation in DNA damage sulting in a ''comet with tail'' formation. Fifty-six in the same areas is likely due to the impact of crop samples, a total of 606 tadpoles, from 18 sites in rotation, including leaving fields fallow, the timing southern Ontario, collected between 1993 and of rainfall, and/or the application of pesticides. R. 1995, were examined. Samples of R. clamitans tad-clamitans tadpoles, especially those collected from poles collected in 1994 and 1995, from regions agricultural areas, also showed significant seasonal with heavy agricultural activity, gave significantly variation in DNA damage. There was no significant higher (P Γ΅ 0.001) DNA length to width ratios than (P ΓΊ 0.05) seasonal or annual variation in the levels samples of R. clamitans tadpoles collected from sites of DNA damage in R. pipiens tadpoles collected in the Bruce Peninsula and near the French River, from the Tallgrass Prairie. This study indicates that which have little or no agriculture. Samples of R. both species are suitable for use in the alkaline SCG pipiens tadpoles collected in 1994 from sites on the assay and as in situ sentinel organisms for environoutskirts of Windsor, Ontario, sites which receive mental biomonitoring. Environ. Mol. Mutagen. genotoxic inputs from nearby industries, gave sig-29:418 -430, 1997. α§ 1997 Wiley-Liss, Inc. nificantly higher (P Γ΅ 0.001) DNA ratios than sam-