Crude extracts obtained from the roots and aerial parts of Rumex crispus L. and Acinos rotundifolius Pers. and the whole plants of Ajuga chamaepitys L. and of Lycopus europaeus L. were evaluated for in vitro antimicrobial activity against five Gram-positive bacteria including Staphylococcus aureus, Micrococcus luteus, Mycobacterium smegmatis, Bacillus subtilis, Bacillus subtilis var. niger, and three Gram-negative bacteria including Aeromonas hydrophila, Klebsiella pneumoniae, Pseudomonas aeruginosa, and the yeast Candida albicans. The inhibition zone diameter was determined for each extract using the agar well diffusion method at a concentration of 12.5 mg/ml. The acetone extracts of the roots of R. crispus and A. rotundifolius demonstrated significant inhibitory effects against most microorganisms under test. © 2005 Taylor & Francis Ltd.