Small fish are increasingly used as organism toxicity models, and one of these, the ten spotted live-bearer fish (Cnesterodon decemmaculatus), has recently been employed in geno- and cyto-toxicity testing. Genotoxic biomarkers are largely employed for monitoring anthropogenic activities, including industrial, agricultural, and urban waste, with clear adverse effects on human health and wildlife populations. The increased attention to the use of non-traditional species of fish in ecotoxicology and genotoxicology research lies in their ability not only to reveal the genotoxic and cytotoxic effects of many potential environmental xenobiotics, but also to help researchers understand the behaviour of real ecosystems. This chapter presents an overview of selected research that has led to the use of C. decemmaculatus as a reliable and valid model of ecotoxicology and genotoxicology in in vivo and in situ studies.