Population of the freshwater amphipod Gammarus fossarum was investigated in a calcareous stream with almost constant temperature (7-8 • C) in the Chočské Vrchy Mts (West Carpathians, Slovakia). Quantitative samples of G. fossarum taken during 2005 showed population densities varying from 100 m -2 in August to 585 m -2 in late November. The population was split into juveniles, mature males, mature females without eggs and females with eggs. The percentage of juveniles (40-64%) was always the highest of any of the categories. Ovigerous females occurred throughout the year. The mean sex ratio was 1 : 2.4 (male: female), although its values varied considerably with the time of year. Breeding was continuous, although juvenile recruitment peaked in early spring, summer and early winter. Three discrete cohorts were distinguished from the size frequency distributions. The life span was 6-7 months and the individuals matured approximately in the half of life cycle. The mean fecundity was 9.6 embryos per brood. Variation in fecundity was mostly explained by size of the incubating females. The absolute growth of this species was best described by the Gompertz growth function. Relative growth rates (% body DM day -1 ) fluctuated in a nonlinear manner with size and age. The highest values of daily growth (2-4% of dry mass per day) were noted approximately in the half of life cycle. Annual production, estimated by the size-frequency method, was 1618.9 g dry mass m -2 and P/B ratio was 5.15.