Antibiotic resistance among anaerobic bacteria has risen sharply in the last thirty years. The most extensive Slovenian research about antimicrobial resistance of anaerobic bacteria from 2015 showed that resistance to amoxicillin/clavulanic acid of the most frequently isolated B. fragilis group was 8%, and the resistance to imipenem was 1%. There is no data about susceptibility for the other beta-lactam antibiotics in combination with beta-lactamase inhibitors as testing for them is not performed in our routine diagnostics. For this reason, we decided to test the susceptibility of selected anaerobic bacteria from the genus Bacteroides and Parabacteroides to amoxicillin/clavulanic acid, piperacilin/tazobactam, and ceftolozan/tazobactam with a gradient diffusion method. We tested a total of 120 prospectively collected clinically significant anaerobic bacteria (104 isolates of the genus Bacteroides, 16 isolates of the genus Parabacteroides) and 87 selected isolates from the IMI collection with known resistance to amoxicillin/clavulanic acid (77 isolates of the genus Bacteroides, 10 isolates of the genus Parabacteroides). The resistance of prospectively collected Bacteroides isolates to amoxicillin/clavulanic acid was 17.3%, and 31.3% for the Parabacteroides isolates. The resistance of isolates of the genus Bacteroides to piperacillin/tazobactam was 11.5%, and among isolates of the genus Parabacteroides resistance was as high as 43.8%. The percentage of resistance of the selected isolates with known resistance to amoxicillin with clavulanic acid was even higher: 58.4% for Bacteroides isolates and 50% for isolates of the genus Parabacteroides. As there are no guidelines for interpretive breakpoints for ceftolozane/ tazobactam, we only determined MIC distribution. For prospectively collected isolates, the MIC50 value was 16 μg/mL, and MIC90 was 256 μg/mL. All selected isolates of the genus Parabacteroides with known resistance to amoxicillin/clavulanic acid had a MIC value of 256 μg/mL, while the value range for isolates of the genus Bacteroides ranged from 1.5 to 256 μg/mL.