The data on size composition and some biological charactors of Illex argentinus in the waters off Argentina were collected by R/V SHINKAI MARU from April 1978 to April 1979. Based on the mean lengths of monthly size compositions, the von Bertalanffy's growth equations were estimated. The equations were ML=26.48 (1-e^{-0.28(t-0.44)}) for males and ML=34.13(1-e^{-0.18(t-0.27)}) for females where ML and t were mantle length in cm and age in month, respectively. It was indicated that this species had a one-year life span, viz. they were born in austral winter to early spring, attained at maturity at the same season of the next year and died after spawning.