An incomplete anoxic fermentation of livestock waste results in offensive odor emissions. Antimicrobial additives may be useful in controlling odor emissions and pathogens. Natural antimicrobial compounds, carvacrol or thymol at 16.75 mM (2.5 g/l) completely inhibited the production of the offensive odor compounds, isobutyrate, valerate, isovalerate, and cresol, and significantly reduced other short-chain volatile fatty acids and gas emissions from swine waste. Fecal coliforms were reduced from 6.3 X 10(6) to 1.0 X 10(3) cells per ml 2 days after treatment with carvacrol (13.3 mM) and were not detectable within 14 days. Total culturable anaerobic bacteria were reduced from 12.4 X 10(10) to 7.2 X 10(8) cells per ml after 2 days and were suppressed below this level for 28 days. Lactate production was not prevalent in untreated swine waste indicating that the microbial populations differ from those in cattle waste. Carvacrol and thymol were stable in swine waste under anoxic conditions for 62 days with 90 to 95% of the additive being recovered in the waste solids. In conclusion, carvacrol and thymol are not metabolized in anoxic swine waste and they are potentially useful in controlling odor emissions and pathogens in swine waste.