Feral swine (Sus scrofa) are abundant throughout the southern United States with a complex legal status and a reputation for negative interactions with wildlife and vegetation. The impacts of feral swine upon water quality are not extensively nor quantitatively documented in the published literature. We quantified the effects of feral swine on dissolved oxygen, fecal coliform bacteria, overall heterotrophic bacteria plate counts, and the presence of disease-causing bacteria. We sampled Mill Creek in western Louisiana in summer 2002 and spring 2003. Feral swine increased fecal coliform counts (P = 0.03 in 2002 and P ? 0.01 in 2003) and heterotrophic plate counts (P ? 0.01 in 2003). Fecal coliform counts (r2 =0.25, P = 0.01 in 2002, r2 = 0.30, P ? 0.01 in 2003) and heterotrophic plate counts (r2 = 0.44, P = 0.02 in 2003) were positively related to swine presence. We also identified pathogenic bacteria, Aeromonas spp., Staphylococcus aureus, and Shigella spp., in swine impacted water. We quantified the increase in fecal coliform bacteria and suggest the potential for disease transfer caused by feral swine. We recommend swine management consider negative impacts of swine on water quality and the potential health hazards of high swine densities on humans and other wildlife.