So far there are no detailed data on the population dynamic of Gammarus balcanicus in the Žrnovnica spring despite its wide range and great importance in matter circulation in streams and rivers. In concordance with that, the aim of this study was to investigate the ecology and some aspects of reproductive biology of the species. Field studies were carried out across the four season from winter 2013 to autumn 2014 at eucrenal of the Žrnovnica spring. Standard physical and chemical parameters of water were measured monthly from December 2013 to December 2015. Quantitative replicate samples (modified AQEM protocol) were collected using a Surber net once per season. In the laboratory, the specimens were classified according to their ontogeny, reproduction status and sex during the each season on various microhabitats. The aim was to examine the range of abiotic environmental conditions during the seasons, population dynamics of the species, and some aspects of life history traits. There were no significant seasonal variations in physico-chemical parameters and temperature which confirmed eucrenal conditions with less than 1 °C of fluctuations. Species Gammarus balcanicus at the spring of the River Žrnovnica prefer fital on which they are prevalent according to their number and size. Males are generally more numerous and larger than females, and ovigerous females and juveniles occur in all seasons. Females show a greater abundance in the warmer months. The breeding period of G. balcanicus is ongoing through the whole year because of constant water temperature at the spring source as well as a result of the influence of Mediterranean climate and higher average annual air temperature.