In this study, the antibacterial activity of Senecio sandrasicus P.H. Davis (Asteraceae), an endemic species of southwestern Anatolia, was investigated. The antibacterial effects of extracts of chloroform, ethanol, hexane, and ethyl acetate, which were obtained from voucher specimens of the plant, were tested by a disk diffusion method against 10 strains of Stenotrophomonas maltophilia . The strains are resistant to various antibiotics including some widely used beta-lactam antibiotics (penicillin and cephalosporins), beta-lactam + beta-lactamase inhibitors, monobactams, carbapenems, and aminoglycosides. The results showed that all of the four extracts of S. sandrasicus had similar potencies against all tested bacteria. The activities were increased dependies on the doses of extracts; 3.6 mu g/disk extracts had higher antibacterial activity than 0.4 mu g/disk. The strains of S. maltophilia , which are important pathogens, were inhibited by some extracts of S. sandrasicus .