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To address the dual needs for improved methods to assess potential health risks associated with chemical exposure in aquatic environments and for new models for in vivo mutagenesis studies, we developed transgenic fish that carry multiple copies of a bacteriophage [Lambda] vector that harbors the cll gene as a mutational target. We adapted a forward mutation assay, originally developed for [Lambda] transgenic rodents, to recover cll mutants efficiently from fish genomic DNA by [Lambda] in vitro packaging. After infecting and plating phage on a hfl- bacterial host, cll mutants were detected under selective conditions. We demonstrated that many fundamental features of mutation analyses based on [Lambda] transgenic rodents are shared by transgenic fish. Spontaneous mutant frequencies, ranging from 4.3 x [10.sup.-5] in liver, 2.9 x [10.sup.-5] in whole fish, to 1.8 x [10.sup.-5] in testes, were comparable to ranges in [Lambda] transgenic rodents. Treatment with ethylnitrosourea resulted in concentration-dependent, tissue-specific, and time-dependent mutation inductions consistent with known mechanisms of action. Frequencies of mutants in liver increased insignificantly 5 days after ethylnitrosourea exposure, but increased 3.5-, 5.7- and 6.7-fold above background at 15, 20, and 30 days, respectively. Mutants were induced 5-fold in testes at 5 days, attaining a peak 10-fold induction 15 days after treatment. Spontaneous and induced mutational spectra in the fish were also consistent with those of [Lambda] transgenic rodent models. Our results demonstrate the feasibility of in vivo mutation analyses using transgenic fish and illustrate the potential value of fish as important comparative animal models. medaka | ethylnitrosourea