The contents of 5 Petri dish, 3-week-old corn meal broth cultures of Pythium ultimum, were homogenized in a Waring Blendor (15 times for 10 seconds each at high speed) with 100 ml water. The blended material was washed through a 100-mesh sieve and centrifuged at 6500 g for 15 min. The supernatant was discarded, and the pellet was resuspended in 20 ml water and mixed with 50 ml of 2.5% agar held at 40 C. Microscope slides were dipped into the suspension and placed in moist chambers to prevent drying. Slides with hardened agar films were placed vertically at a slight angle against the inside walls of 7.5-cm-diameter plastic pots, and soil was packed carefully against the agar surfaces. A cotton seed was planted adjacent to each slide so that primary and/or lateral roots would grow down across the agar film. After various incubation periods at 25 C, aerial portions of the plants were cut and discarded, and slides with a quantity of soil and roots were removed from the soil mass and air-dried. Soil was brushed carefully from the slides with a camel hair brush and roots not adhering to the agar film were removed with a scalpel. Soil was removed from adhering roots and root tips with careful brushing and/or gently flowing tap water. Agar films with adhering roots were air-dried and stained with 0.03% acid fuchsin in 85% lactic acid and examined microscopically. Oospores germinated in the rhizosphere of both primary and secondary roots. Germination was not observed at distances greater than 1.5 mm from the root surface. All germ tubes grew towards the roots. Germination occurred mostly in the root hair zone, but occasionally oospores germinated near root tips. Germ tubes normally made contact with the root epidermis between root hairs or at the bases of root hairs. Only rarely was contact and/or penetration of root hairs observed.