Introduction. Calamus (Acorus calamus L) grows in South and North America and Asia, it reached Europe with the invasions of the Tatars. It is a perennial with strongly branched, creeping, aromatic rhizome, strongly elongated, sword-shaped, pointed leaves. Reaches a height of up to 1 m. It has a cob-shaped inflorescence with green-yellow flowers. The fruits are small red berries, but in our climate calamus does not bear fruit. The extracts and essential oil obtained from the plant contain: ?- and ?-asarone, cyperenol, cyperol, acorin, acoritin, cyperenone, isoazarone, 2,2,5,5-tetramethyl-3-hexanol, galgavin, sacuranin, retusin, isoeugenol, elemicin, safrole, geranyl acetate, spathulenol, borneol, eugenol, linalool, camphor and linoleic, polmitinic and palmitoleic acid. Both the extract and the essential oil have antimicrobial properties. Aim. The investigation activity of calamus oil on aerobic and microaerophilic bacteria. Material and methods. The aerobic and microaerophilic bacteria were isolated from oral cavity and upper respiratory tract. The strains belonging to following genera: Staphylococcus (7), Enterococcus (4), Corynebacterium (2), Acinetobacter (3), Citrobacter (2), Escherichia (4), Klebsiella (2), Pseudomonas (5), Serratia (2), Aggregatibacter actinomycetemcomitans (11), Campylobacter (3), Eikenella (4), Rothia (2) and 7 reference strains were used. Investigation for aerobic bacteria was carried out in Mueller-Hinton agar which contain 2.5-20.0 mg/ml calamus oil (Semifarm) and incubation was carried out in 37°C for 24 hrs and for microaerophilic bacteria in range 0.12-2.0 mg/ml in Brucella agar with 5% sheep blood, and was incubated in anaerostats (CAMPY Pak, BBL) in 37°C for 48 hrs. The inoculums contained 10 5 CFU per spot. The lowest dilution of calamus oil that completely inhibited the growth of the tested bacteria was taken as the MIC. Results. The oil (10.0-15.0 mg/ml) inhibited the grows 4 strains from Staphylococcus aureus genus. MIC for remaining cocci were 15.0-? 20.0 mg/ml. The activity of calamus oil in the case of Gram-negative sticks turned out to be lower (15.0-? 20.0 mg/ml). The oil was more effective against microaerophilic bacteria (MIC 1.0-? 0.12 mg/ml). The strains from Campylobacter genus characterized the most susceptibility to oil (MIC ? 0.12-0.25 mg/ml). Until 81% of strains from Aggregatibacter actinomycetemcomitans genus was susceptible in concentrations ? 0.12-0.25 mg/ml. The rods from Eikenella and Rothia genus turned out to be sensitive to the concentration of the oil in the range of ? 0.12-1.0 mg/ml. Conclusions. The Gram-positive aerobic bacteria were most susceptible to calamus oil then Gram-negative rods. The oil was highly effective against microaerophilic bacteria of the Campylobacter sputorum species. Calamus oil showed higher activity against microaerophilic bacteria compared to aerobic microorganisms.