Abstract: Based on the biological data(n=271)collected in the southern Atlantic Ocean off Argentina during December 2009 to January 2010,the present study analyzed the biological characteristics of rockcod(Patagonotothen ramsayi),including total length(TL),standard length(SL),the relationship between TL and body weight(BW),sex ratio,sexual maturity and size at 50% sexual maturity.The results showed that:(1) The TL ranged from 145 to 302 mm with dominating TL of 200 to 260 mm,the SL ranged from 135 to 265 mm with dominating SL of 160 to 220 mm.The TL(F=1.930,P=0.167＞0.05)and SL(F=1.965,P=0.163＞0.05)distribution between female and male individuals had no significant differences.The relationship between TL and SL was TL=0.875 5SL 0.007 6(R²=0.984 6).(2) The relationship between TL and BW was BW=1.084 5?10^-6 TL^{3.396 8}(R²=0.981 8)and the relationship between TL and BW for female and male individuals had no significant differences(ANCOVA,F=1.755,P>0.05).(3) The ratio of female to male was 1∶0.17.(4) The sexual maturity was dominating of stage Ⅱ,the percentage of stages Ⅲ to Ⅵ was low during December to January.Significant difference can be found for the sexual maturity among TL classes(χ²=569.59,P<0.05).(5)Total length at 50% sexual maturity was 275 mm and the maturation rate was 0.026 3/mm.