Increasing severity of environment associated with increasing altitude in mountain ecosystems (decreasing temperature, increasing duration of snow cover) can affect relative and absolute biomass-allocation patterns in plants. Biomass allocation was examined along a 620-m altitudinal gradient in the Australian Alps for the important environmental weed Achillea millefolium (L., Asteraceae, yarrow). Relative and absolute biomass allocation (dry weight) to reproductive (capitula, flowering stems and associated leaves), vegetative (leaves not on the flowering stem) and below-ground structures (rhizome and roots) were measured at each of nine sites by using quadrats. Increasing altitude resulted in a decrease in relative and absolute allocation of biomass to reproductive structures. For example, the dry weight of inflorescences declined as altitude increased because of a decrease in the weight of the terminal and side branches of the inflorescences, but not the total number of capitula produced. There was also a trend for increased relative allocation to below-ground structures with increasing altitude, even though altitude did not affect absolute allocation to below-ground and vegetative structures, or the total dry weight of A. millefolium. These results are consistent with those for other species growing at high altitude. The management implications for the control of A. millefolium in the Australian Alps are discussed, including in relation to predicted climatic change.