Abstract Allometric growth of chelae dimensions was analyzed to assess the average size at the onset of morphometric maturity (ASOMM) and sexual dimorphism regarding the pair of chelae in the Aegla castro. Both adult males and females are heterochelous with the most robust chela occurring predominantly on the left side. Both chelae are larger in males than they are in females of similar size, thus characterizing these structures as sexually dimorphic traits. The ASOMM estimated for males and females were 10.91 and 10.03 mm of carapace length (CL), respectively. The increase in variability of chelae dimensions in post-pubertal males led to the recognition of two morphotypes (I and II). Temporal variation in the proportions of morphotype II males in relation to females showing late ovarian development was synchronous, making the sexually functional nature of these males evident. The average size at the onset of functional maturity (ASOFM) estimated for males (based on the transition from morphotype I to morphotype II) and females (based on the detection of late ovarian development or eggs) were 17.12 and 12.59 mm of CL, respectively. Ovigerous females were sampled from April through August 2007, characterizing a marked seasonal reproductive period lasting for 5 months.