Considering that carbapenems are used as last-resort medicine in the treatment of infections caused by multidrug-resistant enterobacteria, increasing prevalence of carbapenem-resistant enterobacteria is becoming a growing problem on the global scale. It is considered that discharge of inadequately processed wastewaters could contribute to this problem. The aim of this master thesis was to determine phenotypic susceptibility of 200 enterobacterial isolates, isolated from Zagreb’s communal and hospital wastewaters, to carbapenems and other clinically important antibiotics and to determine dominant mechanism of resistance to carbapenems in carbapenem-resistant isolates. Susceptibilty of isolates to 3 carbapenems (ertapenem, imipenem, meropenem) was tested by microdilution method where most of the isolates, communal and hospital, were resistant to all 3 tested carbapenems. Colorimetric CarbaNP test confirmed that carbapenem-resistance for most of the analyzed isolates was mediated by carbapenemase production. These isolates were mostly resistant to all groups of β-lactam antibiotics and fluoroquinolones, what was determined by disk diffusion method. On top of that, smaller part of isolates (mainly <50%) also manifested resistance to aminoglycosides and sulfonamide-trimethoprim combination, what was determined by disk diffusion method and also resistance to colistin, what was confirmed by microdilution method. Most of the isolates manifested multidrug-resistant phenotype ie. resistance to at least 3 classes of antibiotics, what was more prominent in hospital isolates. Most of these multidrugresistant isolates manifested resistance to carbapenems by carbapenemase production. Results of this research point out hospital and processed communal wastewaters as factors that contribute to the spread of multidrug-resistant enterobacteria in the environment, which presents a risk for human health. Especially worrying is the possibility of spreading enterobacteria resistant to carbapenems and colistin in this manner.