Rhizomorphs ofArmillaria mellea(Vahl ex Fr.) Kummer absorbed ammonium and radio-actively labelled chloride and orthophosphate ions; ammonium uptake was associated with synthesis of amino acids in the rhizomorphs. Uptake of labelled ions occurred primarily at the unpigmented growing tip. Nutrient uptake by rhizomorphs may explain why the dry weight of rhizomorphs is related to the nutrient status of the soil in which they are growing.