SummaryXiphophorw, hellerihas four subspecies living in different water systems of Central America. We have no idea how long they have been geographically isolated, because there could have been temporary communications between the single water systems in the past, even as they may occur today. The four subspecies can easily be distinguished by colour patterns and body shape.Four breeding stocks representing two subspecies and two populations of another subspecies were examined concerning morphology, development of gonads, growth rate, and maturity. ♀♀ and ♂♂ were always treated separately. For investigations of genotypical differences between the four breeding stocks it was necessary to carry out the experiments under constant environmental conditions.Among numerous morphological characteristics only three showed differences which were guaranteed by statistical calculations: 1. number of dorsal fin rays, 2. relation between total length and maximal body depth, 3. relation between total length and depth of peduncule. For all three characteristics a cline from north to south was obvious.Great differences concerning early development of gonads in either male or female direction were not observed between the stocks ofX. helleri. Only inX. helleri strigatusthe formation of gonoduct was slightly retarded in comparison with the other subspecies. This fact is related to late maturity inX. helleri strigatus.The connection between age, size, and maturity when genetically determined is the following: Late maturity is correlated to high ontogenetic age (= size); however, body size does not increase as fast as absolute age, meaning that growth rate is reduced. For other species some authors partly state the contrary. The possibility is established that different species exhibit different connections between maturity, size, and age.The modificability of growth rate and sexual maturity was studied by changing the conditions of space and food. The deterioration of environmental conditions leads to poorer growth rate and later size at maturity. These results correspond to Svärdsons conception; however, Svärdsons curve of modification includes, as well, an extremely favourable environment.The ♂♂ are always divided into two groups: those that show sexual differentiation at a young stage (Frühmännchen, ♂♂F) are tiny, slender ♂♂ which reach maturity and break off growth very early. On the other hand there are those which have a clumpsy, femalelike figure and whose maturity is retarded (Spätmännchen, ♂♂s). The curve of growth rate of these animals is similar to that of ♀♀. ♂♂sand ♀♀ do not stop their growth rate after maturity immediately but reduce their growth rate slowly. The different growth rate of ♂♂Fon one hand and ♀♀ and ♂♂son the other may be explained by different production and different effect of sexual hormones.♀♀ inseminated by ♂♂Fproduce an offspring with a higher percentage of ♂♂ than ♀♀ inseminated by ♂♂S. Inasmuch aspolygenic sex determination can be taken for granted, ♂♂Fand ♂♂Scan be interpreted as sexually strong and weak individuals.In contradiction to frequently occurring statements in literature sexual inversion withinX. hellerinever could be observed in spite of long‐dated investigations. It has become evident that ♀♀‐likeness of ♂♂Sbefore maturity, arrhenoidie‐ and seneacence‐phenomena of ♀♀ have given rise t