Allozymes were used to investigate patterns of clonal growth (vegetative spread) in two closely related multi-stemmed (mallee) eucalypts from Western Australia. Eucalyptus argutifolia Grayling & Brooker, a rare species with a localised distribution, appeared to be extensively clonal. Several populations had putative clones larger than 100 m(2), with the largest clone size estimated to be 306 m(2). By contrast, the more common and widespread E. obtusiflora DC. was far less clonal and had a maximum clone size of 22 m(2). Both genetic and environmental factors may have contributed to these contrasting patterns of clonal growth. When clonality is taken into account, most populations of E. argutifolia had very few individuals (less than or equal to 12 genets) and estimates from this study suggest that the entire species may consist of less than 500 genets.