Reared fish differ from wild fish in several aspects, including morphology, because they are adapted to captive conditions that are totally different from natural conditions. To minimize the influence of the hatchery environment on the morphology of fish, the use of environmental enrichment through the incorporation of natural designs in captivity, has been proposed. In the present study, we performed the physical structuring of fish farming tanks to verify the enrichment effect on the morphology of two species of neotropical fishes: Prochilodus lineatus and Brycon orbignyanus. Each species was subjected to four different treatments over two months: tanks with submersed logs, with artificial aquatic plants, with both structures and without any structure. Results showed that the structural enrichment had a strong effect on the morphology of the cultured fish, which varied with each species analyzed and with the type of structural complexity added to the tanks. There was an increase of morphological variability in the population of P. lineatus and an increase of the average length in the population of B. orbignyanus. This shows that the environmental enrichment is capable to induce morphological differentiation through phenotypic plasticity, probably generating phenotypes more adapted to exploiting a complex environment. Peixes cultivados diferem de peixes selvagens em varios aspectos, incluindo morfologia, pois sao adaptados as condicoes de cativeiro, que sao totalmente diferentes das condicoes naturais. Para minimizar a influencia do meio de cultivo sobre a morfologia dos peixes, o enriquecimento ambiental, atraves da incorporacao de 'designs' naturais em cativeiro, tem sido proposto. No presente estudo, foi realizada a estruturacao fisica de tanques de piscicultura para verificar o efeito deste tipo de enriquecimento ambiental sobre a morfologia de duas especies de peixes neotropicais: Prochilodus lineatus e Brycon orbignyanus. Cada especie foi submetida ao longo de dois meses a quatro diferentes tratamentos: tanque com troncos submersos, tanque com plantas aquaticas artificiais, tanque com ambas as estruturas e tanque sem qualquer estrutura. Os resultados demonstraram que o enriquecimento estrutural teve forte efeito sobre a morfologia dos peixes cultivados, o que variou de acordo com a especie analisada e com o tipo de complexidade estrutural adicionada aos tanques. Houve aumento da variabilidade morfologica na populacao de P. lineatus e aumento do comprimento medio na populacao de B. orbignyanus. Isto mostra que o enriquecimento do ambiente de cultivo e capaz de induzir a diferenciacao morfologica atraves de plasticidade fenotipica, provavelmente gerando fenotipos mais adaptados a exploracao de um ambiente complexo.