The growth of rhizomorphs of Armillaria mellea from small woody inocula through tubes of soil was characterized by a progressive decline in weekly growth increments. Initial growth rate was related to the size of the inoculum food-base; the subsequent decline is attributed partly to depletion of nutrient reserves in the inoculum through fungal respiration and growth, and partly to competition for nutrients between the main growing apex of the rhizomorph and its subordinate branch apices. The vigour of infection of potato tubers by rhizomorphs from such small woody inocula increased with size of the inoculum, and decreased with increasing distance between inoculum and tuber. When inoculum potential of A. mellea was low, the fungus was generally overtaken in the tuber tissues by soft-rotting bacteria, which prevented its further advance.