Isolation and identification of the pathogens are important for appropriate treatment of pleural infections. Distribution of the most frequent causative agents varies throughout the world and may change in time.The objective of the study is to analyze the bacteriological isolates of pleural fluids in order to identify the most frequent infectious agents and assess their variability in time.The retrospective study included 272 patients with positive pleural fluid samples analyzed in 5-year period. The samples were examined using the conventional microbiological technique.Of 315 bacterial isolates the most common were streptococcal species, 105 (33%), of which 55 (17.3%) represented the Streptococcus milleri group. Gram-positive anaerobic cocci were detected in 56 (17.6%) isolates. Enterobacteriaceae family included 27 (8.5%) isolates and Pseudomonas aeruginosa was registered in 13 (4.1%). No statistically significant difference was registered in pathogen distribution in the examined period (p = 0.288).The most common agents of community-acquired pleural infections are the Streptococcus milleri group and anaerobic Gram-positive cocci. They differ from the most common pathogens of pneumonia. Among the hospital-acquired pleural infections, Pseudomonas species, Staphylococcus aureus and enterobacteria prevail. The distribution of bacterial agents isolated in the 5-year period exhibits no significant differences.