Microbiological and chemical quality of drinking water primarily results from water origin and type of applied water treatment. Since the drinking water is one of the mean way through which many infectious agents can be transmitted to humans causing waterborne diseases, constant monitoring of drinking water quality in water supply systems is needed. This study investigates the microbiological quality of drinking water in twenty five public and municipal water supply systems in the area of Osijek-Baranja County in eastern Croatia. The microbiological analyses were conducted for following parameters: total coliforms, Escherichia coli, colony count at 22 °C and 37 °C, enterococci and Clostridium perfringens. In most of investigated water supply systems processed groundwater are used, and since increased concentrations of some chemical compounds in water can influence on appearance and growth of microbiological populations, in this study relevant physicochemical parameters were also measured and correlated with obtained values of analyzed microbiological parameters. Five physicochemical indicators were determined: temperature, turbidity, pH value, free residual chlorine and ammonium concentration. Results indicated that 149 of 1503 analyzed drinking water samples were non-compliance with microbiological criteria set by Croatian regulations (88.7% and 89.2% of 149 had values of colony count at 22 °C and 37 °C higher than the required). Total coliforms, enterococci and Escherichia coli were founded in 3.9%, 1.1% and 1% of non-compliance drinking water samples, respectively. Clostridium perfringens was not detected in any of the drinking water samples. Calculating Pearson’s coefficients of correlation among analyzed microbiological and physicochemical indicators, very weak correlations were obtained. The highest but negative correlation was observed between appearance of microbiological population and concentrations of free residual chlorine (lower free residual chlorine – higher appearance of microbiological population) and positive correlation between turbidity and appearance of microbiological population (higher turbidity – higher appearance of microbiological population).