Rhabdocline parkeriSherwood-Pike, Stone, & Carroll, an endophyte of Douglas-fir, infects healthy foliage by direct penetration of the host epidermal cell wall. Penetration is accomplished by a very fine penetration hypha. Intracellular infections are established in a single epidermal cell and result in the death of the cell. Haustoria lacking neckbands are produced from the intracellular hyphae at the onset of needle senescence. The intracellular hyphae contain large quantities of stored lipid and numerous mitochondria and peroxisomes. Although not growing, the hyphae appear to be metabolically active. Douglas-fir epidermal cells are alive at maturity and contain rudimentary chloroplasts. Small quantities of lipids and starch are present in the healthy epidermal cells. The large central vacuole of the epidermal cells contains primarily condensed tannins and tannin precursors.