Transgenic animals carrying a prokaryotic vector are useful tools for mutation studies and the detection of spontaneous or induced mutations in different tissues. Advances in fish transgenesis make it possible to develop fish that can be used both to assess the health hazards of mutagens in aquatic environments and for comparative mutagenesis analysis. In a paper published online ahead of print in Proceedings of the National Academy of Sciences, Winn et al. report the use of a bacteriophage lambdacII transgene to detect mutations rates (Read the abstract online). The bacteriophage can be easily rescued from the fish genomic DNA and mutated phage are selected by plaque formation on a bacterial lawn. Winn et al. show that spontaneous mutation rates are tissue-specific (higher in liver than in testes) and are roughly comparable with those observed in transgenic mice. Also, mutations induced by exposure to ethylnitrosourea (ENU) are concentration- and time-dependent. This study shows that fish provide a simple lambda-based transgenic model for mutation analysis.