Illex argentinus is a neritic-oceanic squid species,not only supporting the most important world cephalopod fishery in the terms of landing volumes,but also playing an important role in the southwestern Atlantic Ocean ecosystem owing to its high abundance.However,like other annual lifespan Ommastrephid species,it has been found that the recruitment abundance was closely related to the reproductive success the year before.So,in this study,148 females and 99 males of summer spawning stock of I.argentinus were randomly sampled from the catch of Chinese jigger fleet during the fishing season from December 2012 to March 2013,in the high sea of southwestern Atlantic Ocean,and their reproductive biology was studied,in order to fully understand the important reproductive aspects of this species biology.Results showed that the mantle length of this stock during the fishing season was in the range of 172-252 mm for females and 164-248 mm for males.According to the sampled months,the proportion of females decreased monthly,from 84.21% in December to 27.03% in March,but inversely for males,which had the smallest proportion of 15.79% in December,and then increased to the largest proportion of 72.97% in March.Among the females,more than 30% of individuals were mature and some of them were spawning.However,the mature males were in the proportion of more than 60%,nearly twice of that of females,and some of them had mated.With the gonads development,both of females and males grew gradually and attained their largest mantle length and body weight at the functionally maturity stage.The mantle length of females was longer than that of males according to the maturity stages,whereas the body weight of females was lighter than that of males,and the possible reasons were discussed.Moreover,the gonadosomatic index(GSI)for both sexes,nidamental gland index(NGI)for females and spermatophoric complex index(SCI)for males gradually increased with growth,even though the males' SCI was at a same low level during the immature phase.The females' GSI and NGI were in the range of 0.001 2-0.31,with a mean value of 0.074 and 0.003 1-0.55,with a mean value of 0.29,respectively,and both indices attained their greatest value at the functionally maturity stage.The males' GSI ranged from 0.007 5 to 0.084 with a mean value of 0.038,and SCI ranged from 0.000 79 to 0.035 with a mean value of 0.012.These two indices attained their greatest value when the individuals were being mated.