Post-recruit Illex argentinus were collected from the fishery on the Patagonian Shelf between 1986 and 1988. Age was determined by analysis of daily growth increments in ground sections of the statolith, female fecundity was determined, specimens were dissected, weighed and assigned a maturity stage. The relation between mantle length and age is best approximated by a linear model. Both sexes live approximately one year. Females grow faster and attain a larger size than males, but males mature younger. Hatching occurs during the austral winter, peaking in June-July. There is a consistent trend of increasing growth rate with later date of hatching. Statolith growth is negatively allometric with body growth and there is greater divergence between statolith and body growth in females. In post-recruit Illex growth in mass of somatic tissues, apart from the digestive gland, approximates isometry with growth of the whole body. Growth of the digestive gland and the reproductive organs is positively allometric with growth in whole body mass. There is poor correlation between the mass of female reproductive organs and whole body mass, which is due to differences in size at maturity. Mating rarely occurs on the feeding grounds. Fecundity of fully mature females in the sample falls in the range 113835-246098 eggs per individual. This possibly underestimates average fecundity because mature squid on the feeding grounds may be precocious and smaller than average at full maturity. A model combining absolute and relative growth, predicts average male and female growth and growth of the major somatic and reproductive organs. This poorly predicts average female maturity indices with age because of variability in mass of the female reproductive organs. A model that treats maturity stages separately gives close agreement with measured mass of the reproductive organs and maturity indices of a mature female at age one year. Females invest approximately 20% of total body mass in gonad and accessory reproductive organs at full maturity.