A simple cultural method was investigated for its reliability in distinguishing the ericoid mycobiont Oidiodendron maius from selected other species of Oidiodendron. Forty three isolates were grouped by morphology after 28 d growth on cereal agar overlaid with a cellophane membrane. All isolates of O. maius and its close relative O. citrinum expressed characteristic colonial morphologies allowing recognition regardless of sporulation. Isolates grouped by colonial features correlated with

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... y supported groupings obtained by analysis of nuclear ribosomal internal transcribed spacer (ITS) region sequences, including O. maius with O. citrinum, O. griseum with O. flavum, and O. truncatum as an independent group. Isolates of O. tenuissimum, including the ex-type of the purported synonym O. fuscum, demonstrated cultural variation and were dispersed among several different groups in the ITS analysis. O. fuscum is here regarded as a distinct taxon.