Nineteen isolates of Phytophthora spp. and 2 isolates of Pythium spp., most of them selected because they were originally isolated from the host families Solanaceae, Malvaceae and Leguminosae, were evaluated for their ability to cause disease when inoculated into stems of plants of these families or when incorporated into soil into which plants were transplanted. When inoculated into stems, some isolates were extremely host-specific, parasitizing and killing only 1 plant species, while other isolates were parasitic to several. The isolates parasitic to 3 hosts all attacked the same hosts: snap bean, okra and tomato. Plant species also varied in susceptibility to parasitism. Tomato, okra and snap bean were attacked by a number of isolates, while tobacco, pepper, cotton, soybean and lima bean were attacked by 1 isolate or none. In nearly every case, stem inoculation with an isolate produced a brown, necrotic reaction readily distinguishable from the reaction of control plants, which were wounded but not inoculated. Growth of plants transplanted into infested soil was often inhibited by isolates which were unable to parasitize the plants when inoculated by either stem inoculation or soil infestation. Ability to parasitize plants, ability to produce a necrotic reaction in stems without killing plants, and ability to stunt growth of plants transplanted into infested soil were not related. The Pythiaceae apparently possess mechanisms of pathogenesis not dependent on parasitism.