Morphological variation was examined in 19 strains of Aspergillus fumigatus Fres. isolated from the sputum of five patients with aspergilloma. For each patient a series of variant strains was obtained which exhibited gross differences in growth rate, colour and texture of the colony. On microscopical examination, some of the strains were found to produce markedly abnormal fruiting heads which were uncharacteristic for the species. Scanning electron microscopy revealed that the variant produced conidia of anomalous size, shape and surface-sculpturing. The significance of the isolation of morphological variants of A. fumigatus in sputum and their selection in vivo are discussed.