During ontogeny, the allocation of energy can differ throughout the life cycle of a species, especially from juvenile to adult. The present study aimed to describe the relative growth, heterochely, and laterality patterns of the freshwater prawn Macrobrachium brasiliense. Carapace length (CL), abdomen length (AL), left cheliped length (LCL), right cheliped length (RCL), and abdomen width (AW) of males and females were measured. Differences in the morphometric relationships between juveniles and adults were evaluated using analysis of covariance (ANCOVA) and linear regressions. Heterochely was tested using the Mann-Whitney test. Only the relationships LCL vs CL for males and AL vs CL for females were significantly different between adults and juveniles (P < 0.05); these were used to calculate morphological sexual maturity, estimated at 10.3mm CL for males and 8.1mm CL for females. Individuals did not present laterality (P > 0.05) or heterochely (P > 0.05). The average length of the largest and smallest cheliped in males was 9.9 ± 4.8 and 9.2 ± 4.26 mm, while for females it was 7.3 ± 2.4 and 6.8 ± 2.3 mm, respectively. The variation in size and developmental rate of structures suggests that the species follow a mate guarding reproductive strategy since males have well-developed chelipeds to be able to court and guard females during copulation.