The main objective of this study was comparison of the morphological characteristics of the two populations of freshwater snail Ancylus fluviatilis and to determine differences and similarities between measured values. One snail population was sampled from Ruda River (Dalmacija) which belongs to the Adriatic basin and the second population was sampled from stream Slapnica which belongs to the basin of Black Sea. At both sites individuals were manually collected from rocks and pebbles. To determine the morphological characteristics of species, 110 individuals were selected from each of the two populations. For each of the individuals, four morphometric values of the shell were measured (length, height, anterir width, posterior width), as well as their dry and wet mass. Five morphometric ratios of shell were also calculated. Descriptive statistics indicated that the individuals sampled from Ruda River have smaller shells compared to snails from stream Slapnica. According to the results of t-test, all the morphological features and ratios between shell height and shell length, as well as ratios between shell height and shell anterior width were significantly different among two populations. The high value of Pearson correlation coefficient for all measured morphometric features indicated that the correlation was almost perfect. The strongest linear relationship among the measured characteristics of the snail population in the river Ruda was observed between the shell length and shell anterior width (coefficient of determination R² = 0,939). For stream Slapnica the strongest linear relationship was observed between the shell length and its dry mass (R² = 0,862).