A procedure is described for obtaining allometric relationships between stem diameter and above-ground biomass for multi-stemmed trees of the mangroves Rhizophora stylosa and Avicennia marina. The procedure treats each stem as discrete tree that shares a proportion of the butt and other elements common to all stems. Linear log–log relationships were obtained between stem diameter and the dry weights of each above-ground component. Allometric relationships between stem diameter and total above-ground biomass were similar to those for single-stemmed trees in north-eastern Australia, but multi-stemmed trees on the west coast had much greater proportion of their biomass in the form of prop roots than single-stemmed trees of comparable stem diameter on the north-eastern coast. This is attributed to the arid environment on the north-western coast of Australia.