The growth increment and molt frequency of the American lobster (Homarus americanus) ranging in size from 46 to 154 mm of carapace length (C_L) from the southwestern Gulf of St. Lawrence (sGSL) were investigated by tagging and trap studies. The main molting period for lobster in sGSL was from early July to early September. Evidence for two molting seasons was observed only for male lobsters in Malpeque Bay, Prince Edward Island, and Baie des Chaleurs (Northeastern New Brunswick), but there was little evidence of double molting (< 0.2%). Evidence of animals skipping their annual molt was observed throughout sGSL. The percentage of growth increment for lobsters of the same sex tagged in premolt was significantly less than lobsters tagged in postmolt. The lower average percentage of growth increment observed for lobsters tagged in premolt is believed to be related to tagging trauma which is likely greater for lobsters actively developing a new carapace prior to molting. Male lobsters had significantly larger molt increments than females. Comparison between Malpeque Bay and Baie des Chaleurs yielded significant differences in the growth for male and female lobsters but not for berried females. The lowest average percentage of growth increment (12%) was observed for berried females. Furthermore, no relationship was observed between the growth increment and C_L.