Determining the length-length and length-weight relationships and having access to the formulas on the relationships would enable researchers to indirectly estimate the approximate sizes of the organisms when consumed as prey items by examining one of the appendages found in the gut contents. In order to determine some morphometric characters of the Mediterranean green crab (Carcinus aestuarii Nardo, 1847) inhabiting Homa Lagoon, Izmir Bay, Turkey, crab samples were collected using trammel nets, fyke nets, beach seines, and fence traps in monthly intervals between June 2006 and May 2007. A total of 608 male and 559 female specimens were collected during the sampling period. The largest (in terms of carapace length: CL) female and male were 39.59 mm and 51.63 mm, respectively. Morphometric equations for the conversions of length and weight were constructed separately for males, females, and the combined sexes. The equations for carapace width (CW) and right chela width (RChW) for males were found to be RChW = 0.373997 x CW 3.90059, r(2) = 0.85. The relationship between carapace width (CW) and wet weight (WW) was determined to be LnCW = 0.3377 LnW + 2.6942, r(2) = 0.98 for males, LnCW = 0.3424 LnW + 2.6929, r(2) = 0.99 for females, and LnCW = 0.3361 LnW + 2.7019, r(2) = 0.99 for both sexes combined. Males were generally bigger than females. The average values of the carapace width for males and females were 44.58 +/- 7.34 mm and 29.10 +/- 7.07 mm, respectively. The CW/CL ratio for both sexes combined ranged from 1.22 to 1.26.