In field and laboratory experiments the dry weight of Armillaria mellea rhizomorphs produced from woody inocula was proportional to the amount of organic matter in the soil and related to growth rate, number of initials and dry weight per unit length. Addition of nutrient solution or leachate from humus to sandy soil likewise affected rhizomorph growth and branching. Stimulation of rhizomorph growth in soil rich in organic matter did not necessarily increase the flow of N, P or K from the food base. Rhizomorphs from surface soil contained more N and K than those from subsoil. The results indicate that rhizomorphs absorb and utilize nutrients from the soil, that soils rich in organic matter supply more nutrients, and that soil is the principal source of nutrients for rhizomorph growth. In a series of incubation periods, the dry weight of rhizomorphs produced in sand was proportional to the amount of organic matter in the soil of previous incubation periods, suggesting that substances acting like growth factors are absorbed by the rhizomorph-food base system.