Oospores generally have been thought of as relatively resistant survival propagules of many soil-inhabiting fungi of the class Oomycetes. However, as a result of recent emphasis and expanded efforts on biological control of soilborne plant pathogens, oospores have been observed to be parasitized by an expanding array of organisms (Ayres and Lumsden, 1977; Sneh et al., 1977). In earlier studies, Drechsler (1938, 1943, 1952, 1961, 1963) described the invasion of oospores of Pythium species by several Hyphomycetes. Sneh et al. (1977) recently reported that Fusarium oxysporum (Schlecht.) Snyd. & Hans. was frequently isolated from oospores of Phytophthora megasperma (Drechs.) var. sojae Hildb. In this paper we report on: (i) the isolation of Fusarium merismoides Corda [= F. episphaeria (Tode) Snyd. & Hansen] from oospores of Pythium ultimum Trow, (ii) artificial inoculation of oospores with F. merismoides, and (iii) the cytology of the host-parasite relationship involved. Oospores of P. ultimum (isolate P-38) were obtained from 3-4-wkold cultures grown on bean-pod segments in distilled water (Pieczarka and Abawi, 1978). The isolated oospores (and a few sporangia) to be used as trapping baits for antagonistic organisms were sandwiched between two 25-mm-diam polycarbonate-filter membranes (12-,um-pore size) held together by a ring of fast-curing epoxy cement. The oosporemembrane sandwiches were buried 3-5 cm deep in 12-cm clay pots filled with various soils (from snap-bean or table-beet fields from central New York). The pots were maintained in a greenhouse at 19-21 C and were either kept wet (soil moisture above field capacity) or allowed to become dry before rewatering. The oospore-membrane sandwiches were removed from the soils at various incubation intervals over an 8-wk