So far there are no detailed data on the life cycle of G. balcanicus inhabiting the Cetina river 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 reproductive biology of the species. Field studies were carried out from August 2004 to August 2005 at six study sites along the Cetina river. Physical and chemical parameters (water temperature, dissolved oxygen, pH, electric conductivity, alkalinity) were measured and quantitative samples were collected using a Surber net once per month. In the laboratory, the specimens were classified by each month and sampling date in the following categories: adult males, nonovigerous females, ovigerous females, adolescent males, adolescent females, juveniles, intersexes and ovigerous intersexes. The aim was to examine the range of abiotic environmental conditions, their influence on the species' life history traits, and the structure and population dynamic of G. balcanicus in crenal and rhitral of the river. Results have shown that values of abiotic factors have longitudinal gradients typical for streams and rivers. From the spring up to the mouth, the mean water temperature and water temperature fluctuation increase, concentration of dissolved oxygen decreases downstream and its fluctuation increases in the same direction. The lowest pH values were measured at springs. In the limnocrenous Cetina spring, water emerges slower and stays in the pond-like spring for a longer period of time, resulting in higher annual temperature than usually recorded in springs. The greatest population density of G. balcanicus has been recorded at the Cetina spring and reached several thousands of specimens per square meter. A longitudinal decrease in population density was observed. Males were predominant during the whole research period, which can be explained by faster growth and earlier attainment of maturity by males than by females. At the Cetina spring and Obrovac Sinjski study sites, the breeding period of G. balcanicus is ongoing through the whole year because of constant water temperatures at the Cetina spring and the influence of Mediterranean climate on water temperatures along the Cetina river. We recorded the lowest temperatures at which reproduction of G. balcanicus was began The presence of intersex specimens has also been recorded. Intersexes were rare, but with a high ratio of ovigerous intersexes.