Uncharged ammonia is known to cause inactivation of a number of wastewater pathogens, but its effect on Ascaris eggs has never been isolated or quantified. The objectives of this research were to determine the conditions under which ammonia inactivates eggs of the swine Ascaris species, Ascaris suum, and to quantify the impact of ammonia on the U.S. EPA's requirements for alkaline treatment to produce Class A sludge. Eggs were incubated in controlled, laboratory solutions such that the effects of ammonia concentration and speciation, pH, and temperature could be separated. With a 24-h incubation, the inactivation at all pH levels (range 7-11) was not statistically different in the absence of ammonia. The presence of ammonia (0-1000 ppm as N) significantly increased Ascaris egg inactivation at pH 9 and 11, and the ovicidal effect was directly related to the concentration of the uncharged NH3 species. Increasing temperatures (32-52 degrees C) caused increased inactivation at all pH levels and ammonia concentrations. The current EPA treatment requirements to produce Class A biosolids by alkaline treatment have temperature, pH, and time requirements, but do not account for the effectof differences in ammonia concentration on inactivation. To illustrate the potential savings in temperature and pH that could be achieved when accounting for ammonia inactivation, the combinations of ammonia concentration, temperature, and pH neededto achieve 99% inactivation after 72 h were determined. The presence of ammonia at concentrations encountered in sludges and feces (up to 8000 ppm as N) allowed for 99% egg inactivation to be achieved at temperatures up to 14 degrees C lower than ammonia-free controls. Thus, environmentally relevant concentrations of ammonia may significantly increase the rate of Ascaris egg inactivation during alkaline stabilization.