Passive permeability of host cells to the non-electrolytes, methyl and ethyl urea, was measured plasmometrically in epidermal tissue from barley (Hordeum vulgare) inoculated with Erysiphe graminis f.sp. hordei. Incompatibility, conditioned by the Mla gene for resistance, was expressed by hypersensitive collapse of haustorium-containing host cells, 1-2 h after cytoplasmic streaming stopped. Measurements were made about 20 h after inoculation, either before or after the halt in streaming. Before streaming stopped, permeability of haustorium-containing cells to methyl and ethyl urea was reduced about 30%. Permeability to methyl urea was reduced also in neighboring cells without haustoria. Cells in tissues with decreased permeability exhibited high frequencies of concave plasmolysis, indicative of increased adhesion between the plasmalemma and the cell wall. After cytoplasmic streaming stopped, but prior to cell collapse, permeability to ethyl urea was reduced to 50% of values for non-inoculated tissues. The results indicate that structural changes which reduce permeability to non-electrolytes occur in the lipid bilayer of the host plasmalemma and that these changes that may be related to reduced membrane fluidity. These changes are similar in incompatible and compatible host-parasite combinations but occur earlier and to a greater degree with incompatibility.