Monitoring of prevalence and antibiotic susceptibility of strictly anaerobic bacteria, causing infections in hospitalized patients, constitutes a part of a program for prudent use of antibiotics. The aim of the study was to assess prevalence of strictly anaerobic bacteria in patients hospitalized in a tertiary care hospital in 2001-2002 with reference to empiric antibiotic therapy. The most common gram-positive bacteria were Clostridium difficile--27.7%, Peptostreptococcus spp. and Peptoniphilus asaccharolyticus--21.9% and Actinomyces spp.--11.1%. There was an increase in the number of stool samples positive for C. difficile toxins A and B from 39.4% in 2001 to 59.0% in 2002. The results of susceptibility testing of gram-positive isolates showed the highest percentages of strains susceptible to piperacilin/tazobactam--99.6%, ticarcillin/clavulanate--98.5%, imipenem--98.5%, amoxicillin/clavulanate--97.4% and piperacillin--97.4%. The most prevalent gram-negative anaerobes were strains of Bacteroides spp.--43.1%, Prevotella spp.--35.8% and Fusobacterium spp.--11.0%. All tested strains of gram-negative bacteria were susceptible to metronidazole, piperacilin/tazobactam, ticarcillin/clavulanate and imipenem. In the analyzed population beta-lactam antibiotics with beta-lactamase inhibitors, carbapenems and metronidazole may be used in empiric therapy of infections caused by strictly anaerobic bacteria.