Technical Abstract: (Aim) To determine how eugenol affects fermentation parameters and fecal coliforms in cattle and swine waste slurries stored anaerobically. (Methods and Results) Waste slurries (feces:urine:water-50:35:15) were blended with and without additives and aliquoted to triplicate one-liter flasks. Fecal coliforms were eliminated in cattle and swine slurries within one or two days with additions of eugenol at 10.05 mM (0.15%) and 16.75 mM (0.25%), respectively. At these concentrations volatile fatty acids were reduced approximately 70 and 50%, respectively, in cattle and swine waste over 6 to 8 weeks. Additionally, in cattle waste, eugenol stimulated the accumulation of lactate (>180 mM) when compared to thymol treatment (20 mM lactate). In swine waste, lactate accumulation did not occur without additives, but eugenol and thymol stimulated lactate accumulation to concentrations of 22 and 32 mM, respectively. (Conclusions) Eugenol added to cattle waste may be more beneficial than thymol because not only does it effectively control fecal coliforms and odor (VFA production), it also stimulates lactate accumulation. This in turn, causes the pH to drop more rapidly, further inhibiting microbial activitity and nutrient emissions. (Significance and Impact of the Study) Plant essential oils have the potential to solve some of the environmental problems associated with consolidated animal feeding operations. Thymol and eugenol reduce fermentative activity, thus, have the potential to reduce emissions of greenhouse gases and odor, and curtail transmission of pathogens in cattle and swine wastes.