There has been a rapid growth in interest in the development of higher strength, creep-resistant magnesium alloys for the automotive and aerospace industries. All three precipitate phases have been described to form as plates on {l_brace}1{bar 1}00{r_brace}{sub {alpha}} planes of the magnesium matrix phase, and it has been proven difficult to distinguish them on the basis of morphology. These prismatic precipitate plates have been suggested to play an important role in strengthening such magnesium alloys. However, there has been little detailed characterization of these phases and there is a lack of experimental evidence to support the reported precipitation sequence. The role of the metastable intermediate precipitate phase(s) in the nucleation of successive intermediate or equilibrium precipitate phases remains to be established. It is thus the purpose of the present paper to report experimental observations of the precipitation sequence in a WE54 alloy during isothermal ageing at 250 C, with particular attention given to the role of intermediate precipitate phase(s) in nucleating successive metastable and equilibrium precipitation products.