The evolution of microstructure in a cast, extruded, and aged eutectoid Zn-Al-Cu alloy is described based on x-ray diffraction an micrographic observations. Whereas the three phases α′, β′, and η′ were found in the as-cast state, the three phases α, T′, and η′E were observed in this alloy after extrusion at 250 °C. The Al-rich fcc α phase appeared as isolated particles with clear boundaries. The Zn-rich η′E phase decomposed upon aging into α, T′, and η phases. It was found possible to control the phase makeup of the alloy by controlling extrusion temperature. At extrusion temperatures greater than 268 °C, α, E, and η′E were detected, whereas temperatures below this, the phases α, T′, and η′E were found. After prolonged aging, the phase makeup of the alloy was found to be the same regardless of prior treatment routes. Early shrinkage of the extruded alloy occurred after aging at 91 and 150 °C and was related to precipitation of aluminum from the η′E phase during its decomposition to α, T′, and η products.