Instituto de Ciências Biológicas (ICB), Universidade Federal do Rio Grande (FURG), Av. Itália Km 8, Rio Grande, RS, 96208-060, Brazil. Email: julianozanette@furg.br; biozanette@hotmail.comThe recent advances in molecular biology and biochemistry have led to important discoveries and new insights in the fi eld of aquatic toxicology. Fishes that are used in laboratory (e.g., zebrafish Danio rerio) and environmental studies (e.g., North American killifi sh Fundulus heteroclitus) are considered model organisms for such purposes. This chapter aims to review the current literature regarding some genes and proteins involved in the biotransformation of organic molecules in well-known model fi shes, and exploring how the new discoveries could be important and applicable to a wide range of South American tropical fi sh, with a major focus on the study of cytochrome P450 of family 1 (CYP1s) gene/proteins and cyprinodontiforme fi shes.