The morphology, microstructure and composition of the precipitates produced in the decomposition of a Ni-12 at.% Be alloy aged at 843K for several hours have been characterized by tomographic atom probe (TAP), field ion microscopy (FIM) and transmission electron microscopy (TEM). The precipitates are thin, plate shaped and nearly stoichiometric -NiBe with B2 structure. In the samples aged for 5-7h most of the -plates have a thickness of the order of 1nm and an aspect ratio of about 10. The habit plane of the -plates is mainly observed to be parallel to the (001) plane of the matrix. However, some of the -plates show a considerable deviation from this orientation relationship. The interfaces between precipitates and matrix are both structurally and compositionally very sharp. The observed morphology is explained in terms of the strong anisotropy of the interfacial energy, the elastic energy set up during the precipitation, and the ledges at the interfaces.