The maturity and reproductive cycle of female American lobsters (Homarus americanus) were investigated in the southern Gulf of St. Lawrence (sGSL), Canada. The onset of sexual maturity of female lobsters can be established by observations of the ovarian condition, either color or weight, and staging of cement glands but cannot be detected by the morphometry of their abdomens. Females reached 50% maturity between 68.7 mm and 73.3 mm carapace length (L_C). There was a significant geographic difference (P < 0.005) in the size at 50% maturity established by the ovarian development techniques but not by the cement-gland staging technique. Also, there were no annual significant differences (P > 0.005) between the ovarian development techniques used at a single site between 1994 and 1997. To study the reproductive cycle of females, molt stage, ovarian development, and egg spawning were monitored by dissections at the laboratory and by tagging studies in the field. The majority (80%) of small mature females (L_C < 120 mm) in the sGSL had a typical two-year reproductive cycle with molting (with copulation) and spawning in alternating years. However, up to 20% of multiparous females ranging between 65 mm and 109 mm L_C could spawn in successive years instead of the generally accepted two-year cycle, and some could even molt and spawn during the same summer. Similarly, up to 20% of primiparous females could molt and spawn (for the first time) in the same year instead of spawning the following year. A small percentage (5%) of small mature females could also skip molting or spawning for a year. Temperature data suggested that the length of the female reproductive cycle, and possibility of molting and spawning in the same year, were related to the number of degree-days in a particular season.