Pythium graminicolaSubrm. entered barley roots within 2 h after inoculation, and sporangial development began immediately. By 10-14 h after infection most root cells were filled with sporangia. For the first 2-3 days the fungal thallus was mostly sporangial and in the cortex; later it consisted of sexual bodies mainly in the stele. Host cell walls were penetrated rapidly, and chemical breakdown at the place of penetration was obvious. A rim of distorted wall material, caused by lateral expansion of infection hyphae, occurred around the penetration hole on the side of the wall approached by the fungus.