The removal of viruses from drinking-water supplies is not a long-established practice, but it is an issue of major concern. Diatorhaceous-earth filtration has been demonstrated to remove viruses from water effectively; however, as this article emphasizes, more research of and experimentation with this process is necessary. Part 1, involving the study of the bacteriophage T2 for E. coli, appeared in JOURNAL AWWA, 66:2:98 (Feb. 1974).