abstract essential oil from the tubers of cyperus rotundus, obtained by steam distillation, was analyzed by gc and gc/ms. in total, 33 compounds were identified. the oil was characterized by its high content of sesquiterpenes with cyperene (30.9%) being major. the antibacterial activity of oil from tubers of cyperus rotundus, showed more important activity against gram-positive bacteria specially staphylococcus aureus than gram-negative bacteria. the antimutagenic activity was tested by the "sos chromotest" and the "ames" test. c. rotundus oil acted as an antimutagen against afl atoxin b1 in both salmonella strains (ta100 and ta98) and escherichia coli strain (pq37) and against nifuroxazide in escherichia coli strain (pq37), where its mutagenicity is not expressed. the highest rates of afb1 mutagenesis inhibition tested by ames assay, ranged from about 82.56% for ta100 strain to 85.47% for ta98 strain at the same dose of 50 g afb1 per plate. whereas, the mutagenic effect of respectively nifuroxazide and afb1 (50 g/assay) were reduced by aproximately 58.19% and 81.67% when tested by the sos chromotest assay.