Water temperature is an important modulator of variation of phenotype in poikilothermic fish and therefore of body shape. The present study explores the phenotypic response of commercial O. mykiss parr when they are subjected to rearing temperatures of 8º and 16ºC under controlled conditions with the aim of demonstrating, with the use of Landmark-Based Geometric Morphometrics, the influence of rearing temperature on body shape. O. mykiss males and neomales from eggs to parr stages, belonging to the same family and strain, were reared at different temperatures (8ºC and 16ºC) and grouped according to: Group 16ºC/neomales (n = 15), Group 16ºC/males (n = 15), Group 8ºC/neomales (n = 15) and Group 8ºC/males (n = 15). Differences in body shape among sex and rearing temperatures were examined by geometric morphometrics and traditional morphometry. At 16 weeks, geometric morphometric analysis revealed a dimorphism, which was not detected by traditional morphometry, mainly in the dorsal and caudal fin area. This dimorphism was not observed in neomales. This difference may be the product of the potential for phenotypic plasticity and adaptation to genetically controlled environmental and local fluctuations, which affects to indicators of adaptation in unfavorable environmental conditions such a biomechanical action of swimming and propulsion. In conclusion, this study demonstrates the differential effect of 8º and 16ºC water temperature on lateral body shape in male O. mykiss parr during the early stages of body development and in controlled conditions.