Based on the biological data (n=168) collected in the southern Atlantic Ocean off Argentina during December 2009 to January 2010, the present study analyzed the biological characteristics of Southwest Atlantic butterfish (Stromateus brasiliensis) in the Austral Summer, including total length (L_T), fork length (L_F), the relationship between L_T and W_B, sex ratio, sexual maturity and gonadsomatic index. The results showed that: (1) The L_T ranged from 23.6 to 34.2 cm with dominating L_T of 24.0 to 28.0 cm, the L_F ranged from 21.6 to 30.3 cm with dominating L_S of 22.0 to 26.0 cm. The relationship between L_T and L_S was L_T=1.141 1L_F-6.770 2 (R²=0.962 6). (2) The relationship between L_T and W_B was W_B=3×10^-5 L_T^{2.878 9} (R²=0.924 4). (3) The ratio of female to male for Southwest Atlantic butterfish was 1∶0.3. (4) The sexual maturity of Southwest Atlantic butterfish were Stage Ⅱ or Ⅲ during December to January. Significant difference can be found for the sexual maturity of Southwest Atlantic butterfish among L_T classes (χ²=104.86, P<0.05). (5) The gonadsomatic index was increased with the increasing of fork length and the gonadsomatic index ranged from 1.05 to 8.33.