Specimens of Illex argentinus from trawl catches along southern Brazil were sampled between March 2007 and January 2008 to study aspects of their life cycle and the connectivity with stocks from the southern range of the distribution of the species. Small mature specimens occurred year-round, whereas those with a mantle length (ML) of more than 250 mm were rare in the warm months, from December to May. More than 79% of the mature females were mated, providing evidence of year-round spawning in southern Brazil. Mature males were 131–330 mm and 147–275 days old, and females were 166–341 mm and 146–257 days old; mean ML at 50% maturity was 156.3 mm and 181.2 mm, respectively. The number of daily increments decreased from the capture date showed that hatching occurred year-round, mostly from August to January. Daily mean ML increment of juveniles to adults hatched during the warm season and caught during the cold season was larger than that of those hatched during the cold season and caught during the warm season, suggesting greater growth in winter and spring associated with increasing primary productivity in the region. A life cycle shorter than 1 y suggests genetic isolation neither between both seasons nor among larger and smaller adult squids. The number of daily increments in the postnuclear zone of the statoliths was substantially lower than that recorded for specimens maturing along Patagonian waters, suggesting that those squid do not reach southern Brazil.