In this study, the data on body size, habitats and abundance of syntopic Emys orbicularis and Mauremys rivulata from Great Menderes Delta, Aydin, Turkey were presented. A total of 20 (12 males, 4 females, 4 juvenile) E. orbicularis and 69 (24 males, 14 females, 31 juvenile) M. rivulata were marked during five sampling periods from June 2010 to July 2011. Seven of M. rivulata and one of E. orbicularis were recaptured. The population size was estimated to be 48 +/- 10 individuals for E. orbicularis; 341 +/- 110 individuals for M. rivtdata. The survival rate was also calculated as 0.93 (0.88 - 0.96) and the capture probability was estimated as 0.67 (0.44 - 0.84) for M. rivulata. The mean straight-line carapacial length (SCL) was 111.1 mm for adult males, and 134.5 mm for adult females; mean adult body mass was 217 g for males, and 418 g for females of E. orbicularis. Mean SCL was 112.6 mm for adult males, and 139.6 mm for adult females; mean adult body mass was 174 g for males, and 411 g for females of M. rivulata. Sex ratios were male-biased for both species (3 for E. orbicularis, 1.7 for M. rivulata). In addition, factors threatening the local population were discussed.