A comprehensive study of sex differentiation gonochoric and hermaphroditic fishes, this booka examines sex chromosomes and sex determining genes, emphasizing the need to search for more than one sex-determining gene residing on different chromosomes. Ita traces the origin of hermaphrodites from secondary gonochoresa and explains the conservation of homologous sex differentiation and steroid receptor genes among gonochores and hermaphrodites. The author identifies the optimal broodstock size and the appropriate candidate species for bait. He showsa that the analysis of steriles and mutants has demonstrated the value of fishes as a system for genetic dissection of human development and diseases.