Two fundamental questions concerning vertebrate sexuality are what controls the stability of gender throughout the life of an individual and what are the genetic, immunological, and endocrine factors controlling sex determination and differentiation. Significant aspects of both issues cannot readily be examined experimentally in standard laboratory vertebrates partly because these species are gonochores and provide no opportunity to examine plasticity in sexual systems, and partly because few means have yet been found experimentally to manipulate immunogenetic factors, such as sex-specific DNA or cell-surface antigens, thought to be involved in the determination and differentiation of sex. Behaviorally induced adult sex change in hermaphroditic fishes constitutes a unique system of controlled sexual plasticity in which these issues can be addressed. Several species of tropical marine fishes satisfy the requirements for an appropriate laboratory experimental animal. Sex change in them is known to involve alterations in external coloration, behavior, gonadal structure, hormonal enzyme activity, levels of circulating steroid hormones, and concentrations of H-Y antigen. Genetic, immunological, endocrine, and behavioral factors interact with one another causally in ways permitting relatively simple manipulations simultaneously to analyze the network of sequential causes for sex change and to address the above-stated fundamental issues concerning sexuality generally in vertebrates. Since sex change can be started at will, these fish species become powerful model systems for the analysis of basic mechanisms of sex determination, differentiation, and long-term stability.