Narrow-clawed crayfish (Astacus leptodactylus Eschscholtz, 1823) within the family Astacidae is a native European species that inhabits lakes and slow streams. The objective of this study is to examine population characteristics of narrow-clawed crayfish in Treće jezero of park Maksimir where its presence has been preliminary recorded as well as determine if there are differences in morphometric characteristics, condition, injury percentage and life cycle phases between sexes and between size classes. Sampling took place from 7th till 16th of June 2016, by both traps and hand. 153 crayfish were caught, their sex was determined, 25 morphometric characteristics per unit were measured, visible injuries, life cycle phases and cement gland activity (in females) were recorded. Population density was determined with CPUE (catch per unit effort) and mark - recapture method. Analysis has shown significant difference in 8 morphometric characteristics between sexes, and its lack in condition and injury percentage. Population of narrowed-clawed crayfish was divided into 6 size classes. Results of variance analysis of morphometric characteristics showed that males siginificantly differ in all parameters between classes while females differ in all, except in areolal width. There was no significant difference in condition between classes, but it was recorded in injury percentage (larger individuals have more injuries than smaller ones). Relative population density for investigated location using CPUE is 0.10 units/trap/night and 15.61 units/person/hour (23 units/m2). Population size was estimated by Schnabel (2303 units) and Schumacher - Eschmeyer (2279 units) method. The study has shown that narrow-clawed crayfish of Treće jezero make a large, stable and viable population under predator pressure. Since narrow-clawed crayfish have characteristics of invasive species, it is necessary to continue monitoring population characteristics of this species in the area of park Maksimir and try to prevent it from spreading.