Some biological aspects of Melicertus kerathurus, a shrimp of high commercial value, were studied in Izmir Bay from April 2004 to December 2004. A seasonal reproduction pattern was observed from April to September with a high percentage of mature females occur-ring between May and July, when water temperature was relatively high. During the study, a total of 1050 specimens was captured: 612 (58.3%) females and 438 (41.7%) males. The largest female and male were 22.7 cm and 17.0 cm, respectively. The minimum size attained at first maturity was estimated to be 3.9 cm carapace length (CL) and 14.1 cm total length (TL). Sexual differences related to the length-weight relationship were noted. The onset of sexual maturity for females was estimated to be CL50% = 4.6 cm and TL50% = 16.5 cm.