This study aims to investigate some biological characteristics of Atlantic bonito, Sarda sarda Bloch, 1793, from the Turkish coasts of the Black Sea and the Sea of Marmara. A total of 212 S. sarda (100 females, 89 males, and 23 undetermined) were sampled monthly between May 2011 and September 2012. The sex ratio (male/female) was 0.89; females prevailed but not statistically significantly so. Fork lengths of all sampled individuals ranged from 17.7 to 63.0 cm. The length-weight relationship for all individuals was calculated as W = 0.01 L-3.085. The ages of the fish ranged from 0+ to 2+. Growth parameters calculated according to the von Bertalanffy growth equation were L infinity = 69.565 cm, k = 0.439, t(0) = -1.327 for females; L infinity = 74.603 cm, k = 0.364, t(0) = -1.518 for males; and L infinity = 67.883 cm, k = 0.463, t(0) = -1.220 for both sexes. One hundred ovaries were obtained from the females, and these ovaries were histologically examined to determine the reproductive conditions and developmental stages of oocytes. The gonadosomatic index values calculated for females indicated that spawning generally occurred between May and August. The most intensive spawning period was observed in June and July. The length at first maturity was estimated to be 42.5 cm for females and 36.8 cm for males.