In a previous study, we demonstrated that tadpoles 0.01) added variance component among tadpoles are suitable organisms for monitoring small bodies for DNA damage and there were significant inof water (e.g., creeks, ponds, and drainage ditches) creases (P õ 0.05) in the length:width ratios of the for genotoxicity using the alkaline single-cell gel DNA patterns at concentrations as low as 1.56 mg/ DNA electrophoresis (SCG) or ''comet'' assay l. However, tadpoles in the last phase studied (both [Ralph and Petras, 1997]. This approach involves pairs of limbs present) showed no significant (P ú detection, under alkaline conditions, of cell DNA 0.05) added variance component and no signififragments which on electrophoresis migrate from cant increases (P ú 0.05) in DNA damage upon the nuclear core, resulting in a ''comet with tail'' exposure to any of the MMS doses tested. A nested formation. In this initial study, most of the tadpoles ANOVA indicated that, for each of the tested concollected were in the early stages of larval develop-centrations of MMS, but not the dechlorinated water ment, but this is not always possible. The present control, there was significant heterogeneity (P õ study evaluated the sensitivity of tadpoles, at differ-0.05) in DNA damage when tadpoles of all four ent stages of larval development, to a range of con-phases studied were compared. However, when centrations of the genotoxicant methyl methane-tadpoles of the last phase of development were resulphonate (MMS). Four specific phases of Rana moved from the comparison, there was no significlamitans (green frog) larval development were ex-cant heterogeneity (P ú 0.05) among tadpoles of amined: first-year limbless tadpoles (Stage I as de-the remaining three phases. Possible reasons for fined by Taylor and Kollros [1946]), second-year this insensitivity to MMS as animals enter the metalimbless tadpoles (Stages II-III), second-year tad-morphic climax were considered. The results indipoles with only hindlimbs (Stages X -XVIII), and sec-cate that pooling of the early tadpole phases of ond-year tadpoles with all four limbs evident and a R. clamitans for SCG environmental genotoxicity tail undergoing resorption (Stages XXII -XXIII). biomonitoring is acceptable. Environ. Mol. Muta-Twenty-four hour exposures to MMS of tadpoles in gen. 31:374-382, 1998. ᭧ 1988 Wiley-Liss, Inc. the three earliest phases produced a significant (P õ