Soil-transmitted helminth infections when light-to-moderate usually are well tolerated, but heavy-to massive infections invariably cause disease. A massive infection with Ascaris lumbricoides may cause intestinal obstruction, liver abscess, or some other condition requiring surgical treatment; more regularly, however, ascaris disease is a form of malnutrition. Trichuris trichiura causes diarrhea and dysentery and, at times, rectal prolapse. The hookworms, Necator americanus and Ancylostoma duodenale, cause blood-loss from the intestine resulting in anemia. Necator infection is acquired percutaneously, and is more frequently massive than is that of Ancylostoma which may be acquired percutaneously or orally. Estimates of egg output in the feces, based on egg-counts by dilution, direct smear, or thick-film techniques, provide a reliable index of light, medium, or heavy infection. Acquisition of heavy infection with Ascaris and Trichuris depends on favorable qualities of the soil, and on the sorting action of rain which transports and concentrates the eggs of helminths in locations where survival and transmission are favored. The high frequency of heavy hookworm infection in southeastern United States and probably elsewhere may depend largely on the presence of feces-burying dung beetles. Human infection with soil-transmitted helminths of dogs and cats has become a serious public health problem attributable to the persistence of rural mores in the urban setting.