It is accepted that the cooling rates of alloys during solidification can strongly influence their microstructure and, consequently, their corrosion resistance and mechanical properties. However, the influence of the cooling rate on the dendrite arm spacings of alloy AZ91 is not often discussed in the literature, and then only within limited solidification conditions for specific casting methods. In the present work, the microstructure of as-solidified AZ91D was characterised for an extensive range of cooling rates. Casting methods such as plaster moulding, permanent moulding and die casting were used to obtain slow and moderate cooling rates. Centrifugal atomization and laser surface remelting were used to obtain higher cooling rates. A relationship was established between the average cooling rate through the solidification range and the secondary dendrite arm spacings.