The inactivation rates of digestion- resistant parasite eggs in laboratory-stored sludge were measured to determine their potential fate in sludge lagoons. Eggs from roundworms (Ascaris, Toxocara, and Trichuris) and a rat tapeworm (Hymenolepis) were added domestic sludges either at the beginning and during, or after aerobic or anaerobic digestion. Digested sludge samples seeded with the parasite eggs were stored in the laboratory at 4ÀC, at 25ÀC, and in a container that was inserted in the ground to simulate the conditions of a sludge storage lagoon. Nonsludge soil samples (controls) were seeded with the same parasites as the digested sludges and stored under similar conditions. The total number of eggs recovered from the samples decreased as storage time and temperature increased. The number of viable eggs and the potential infectivity of recovered Toxocara and Ascaris egg were related primarily to storage temperature and time. After 25 months of storage at 4À C, the Toxocara eggs and some Ascaris eggs remained both viable and infective, whereas most of the eggs stored at 25ÀC were nonviable after 0 to 16 months of storage in sludge. Though storage temperature and time were the most important factors in the inactivation of these eggs, minor effects were also associated with other factors - type of sludge digestion, timing of egg addition (during or after sludge digestion), type of storage (in sludges versus in soil), pH, and species of egg. These controlled laboratory studies suggest that sludge lagooning can be an effective method for inactivating parasite eggs, particularly in warm geographic locations.