Physicians' prescribing practices impact greatly on rational drug use and expenditure. The aim of this cross-sectional study is to determine primary health care physicians' knowledge on rational antibiotic use in Izmir. A total of 475 physicians from primary health care centres (response rate: 100%) participated in this study. Prescription information of physicians was collected with a standardized questionnaire. It was detected that 48.4% of the physicians have prescribed antibiotics in one of each three (30%) prescriptions, while 19.4% in one of each five prescriptions. The most frequent reason for antibiotic prescription was the upper respiratory tract infections (URTI). The rate of prescribing antibiotics for URTI according to the results of throat culture or blood count was 11.8%. The most commonly prescribed antibiotics were penicillins for URTI, quinolones for urinary tract infections and trimethoprim-sulphametoxazole for gastrointestinal tract infections. The rate of appropriate prescribing practices was high among physicians in the age group 25-29 in comparison to older age groups (X-2: 14.855; p: 0.01). Only 32.2% of the physicians correctly indicated the antibiotics with drawbacks in newborn period. 6.5% of the participants gave no correct antibiotic choices for any of the infections indicated, It was stated by 89.6% of the participants that they would benefit from continuous education on rational antibiotic use. The data obtained revealed that primary health care physicians who had high antibiotic prescription rates, lacked information about rational antibiotic use and continuous professional education and development programmes related to this topic is a prerequisite.