Human enteric viruses occur in large numbers in sludges and in aquatic sediments from polluted waters. Of these agents, rotaviruses are often involved in water- and food-borne outbreaks. They represent health hazards of undetermined magnitude. Virus levels in sludges may be reduced, but the viruses are not eliminated by aerobic and anaerobic treatment. A study of the removal of indigenous rotaviruses during the primary settling and activated sludge treatment of raw sewage was conducted for a period of 8 months in a Houston, TX, plant treating 5.55 million L per day. An average rotavirus reduction of 44 to 55% was obtained by primary settling and a reduction of 93 to 99% was achieved in the final chlorinated effluents. Composite sampling at 1-h intervals over a 24-h period indicated average removals of 85%—a measure much more accurate than the misleading average of 6% indicated by one series of grab samples of raw sewage and effluent collected simultaneously. Quantification of rotaviruses was based on counts of immunofluorescent foci 24 h after addition of sample concentrates to coverslip cultures of fetal rhesus monkey kidney cells. Rotavirus quantities ranged from 40 to 510 per liter of raw sewage and from 0 to 25 per liter in the final chlorinated effluent.