Abstract A description is provided for Aphanomyces euteiches . Information is included on the disease caused by the organism, its transmission, geographical distribution, and hosts. HOSTS: Pea, Arabis , pansy, sweet pea, clover, bean, lupin, vetch, lucerne, Melilotus , barley, oats, Echinodorus brevipedicellatus . Conifer seedlings and other hosts were infected by inoculation. DISEASE: Root rot of pea. GEOGRAPHICAL DISTRIBUTION: Asia (Japan); Australia (Tasmania); Europe (UK, Denmark, France, Norway, Sweden, USSR); N. America (USA). (CMI Map 78, ed. 3, 1977). TRANSMISSION: Soil-borne, persisting in the soil for many years. Studies in Wisconsin suggested that A. euteiches may live as a weak parasite in the roots of many plants and occur naturally in some virgin soils (6, 523). Oospores were indicated to be the primary inocula for new outbreaks of pea root rot, zoospores the primary infective agents (39, 646). Survival between pea crops depended on oospore durability and possible alternative hosts, not saprophytic activity (41, 689). Studies were made of population dynamics in the soil (48, 2067) and penetration and infection of roots by zoospores (42, 287).