Technical Abstract: Wastes generated from the production of cattle and swine in confined facilities creates the potential for surface and ground water pollution, emission of greenhouse gases, transmission of pathogens to food and water sources, and odor. It is our hypothesis that something which inhibits microbial fermentation in livestock wastes will be beneficial to solving some of these environmental problems. Our work has concentrated on the use of antimicrobial plant oils, thymol and eugenol. Anaerobic one-litre flasks with a working volume of 0.5 L cattle or swine manure were used to evaluate the effect of thymol and eugenol on production of fermentation gas, short-chain volatile fatty acids, lactate, and bacterial populations. Either oil at 0.2% in both wastes essentially stopped all production of gas and volatile fatty acids, and eliminated all fecal coliform bacteria. In cattle but not swine waste, thymol prevented the accumulation of lactate. However, eugenol stimulated lactate formation in cattle and swine wastes. Thus, eugenol may offer a distinct advantage over thymol, because lactate accumulation in the wastes causes the pH to drop more rapidly, further inhibiting microbial activity and nutrient emissions. Thymol and eugenol were more effective than four commercial additives consisting of various combinations of plant oils or terpenes in controlling fermentation activity in cattle waste. Results from field studies in cattle feedlot pens indicate plant oils must be incorporated into a granular form or something similar to prevent volatilization or degradation of the oil. We conclude that plant oils may offer solutions to controlling various environmental problems associated with livestock wastes, assuming they are cost-effective.