Study on the size at maturity of male American lobster (Homarus americanus) has traditionally been done based on a linear relationship between chelae size vs. body size assuming that a sharp change in slope occurs at onset of maturity. We analyzed this sexual dimorphism by standard log-linear transformations and bivariate allometric plots as well as by multivariate Principal Components Analysis (PCA) in order to verify the validity of this approach for determining size at maturity. These analyses showed that onset of maturity cannot be detected efficiently by claw morphometry because this secondary sexual character gradually changes from early juvenile stages, contrary to what has been believed. Observation of the presence of spermatozoa in the vasa deferentia (gonadal maturity) showed that 50% gonadal maturity occurred at 49.8 mm of carapace length for male lobster from the southern Gulf of St. Lawrence. However, gonadal maturity does not necessarily correspond to the ability to mate (behavioral maturity) in many crustacean species. Therefore, further studies are needed to assess the behavioral maturity to determine the size at maturity for male lobsters.