The Fish Genetics Section of the American Fisheries Society established its Nomenclature Committee to develop and promote standardized genetic nomenclatures. Here, following public comments on previously published draft guidelines, we present the committee's revised version of a nomenclature for protein-coding loci in fish. This nomenclature closely parallels the one used for human genetics, but improves on it in several respects. The fish system (1) includes standardized abbreviations for commonly analyzed proteins, and provides formal symbols for gene loci encoding these proteins; (2) specifies typographic conventions for distinguishing between genes and proteins and for identifying alleles; (3) provides for multilocus isozyme systems, isoloci, regulatory loci, and pseudogenes; (4) allows important basic information (such as subcellular distributions of gene products, active substrate isomers, recent gene duplicates, and orthologous relationships among loci) to be specified in gene symbols via prefixes, suffixes, and multilocus designations; (5) provides three alternative systems of allele designation and distinguishes alleles having quantitative effects; and (6) includes guidelines for specifying genotypes, linkage relationships, and biochemical phenotypes. This nomenclature is intended to be comprehensive, flexible, unambiguous, and applicable to all fish species as well as to other organisms. The American Fisheries Society has adopted this system as an editorial standard for its publications.