The apparently diminished capacity for N2 fixation by the shrub legume Calliandra calothyrsus (Calliandra) relative to other woody perennial legumes was investigated in a field experiment in northern Queensland, Australia. In this trial, (i) the proportion of plant nitrogen (N) derived from symbiotic N2 fixation (%Pfix) and the amounts of N2 fixed were compared in Calliandra, Gliricidia sepium (Gliricidia) and Codariocalyx gyroides (Codariocalyx), (ii) variations in N2 fixation due to season or tree age were determined, (iii) estimates of Pfix derived with the 15N natural abundance technique were compared with values obtained from 15N enrichment or xylem sap ureide procedures to determine whether the previous conclusions about Calliandra's ability to fix N had resulted from specific problems with the natural abundance methodology used in the earlier studies.