Low-stress creep behaviour of a two-phase Zr-2.5%Nb alloy, differently heat treated, has been investigated using helical test specimens. The phase diagram of the alpha (hcp) + beta (bcc) alloy is characterized by the monotectoid reaction at 893 +/- A 10 K: beta (1) -> (alpha + beta (2)) where beta (1) (Zr-20Nb) and beta (2) (Zr-80Nb) have widely differing compositions. At the creep testing temperature, 818 K, which is close to but below the monotectoid temperature, the creep rate for samples with the equilibrium alpha + beta (2) structure has been found to be considerably higher, over an order of magnitude, than that in samples with the metastable alpha + beta (1) structure. Microstructural changes accompanying the markedly enhanced creep rate for the alpha + beta (2) structure at stresses as low as 1-4 MPa have been explained in terms of the relative stability of the beta (1) and the beta (2) phases during the creep process. An attempt has been made to elicit the likely mechanism underlying the observed enhancement of creep rate and the changes in morphology, composition and volume fraction of the beta phase.