The mark-recapture technique for closed populations was employed to estimate the population size and density of Astacus leptodactylus during August and September, 2005 by using minnow traps of 34 mm mesh size in Egirdir Lake. A total of 600 minnow traps were set randomly along the shoreline at approximately 3, 5 and 7 m depth. The nets were set in the late afternoon to each study depths, and were hauled the next day or after two days. The research was performed two times each month. In August, 1956 adult crayfish and in September, 2756 adult crayfish were marked by cauterization of the carapace. The recapture rates were found to be 3.5% in August and 2.3% in September, respectively. A total of 200 crayfish were randomly selected, 74 females and 126 males. The sex ratio was 1:1.7. Moreover, length and weight data gotten from 200 untagged crayfish showed that females and males differed significantly in their weight, but no significant difference was evident in the carapace length. From the mark-recapture experiment, it was estimated that catchable population size ranged between 32590 and 73503 individuals (with carapace length above 45 mm). Estimated density ranged between 0.54 and 1.2 individuals per m(2). Recapture rates did not differ statistically among occasions. Despite the moderate and fluctuated growth in the crayfish population, catchable population should be estimated for each region in the lake by using the estimation methods.