The microstructure of the eutectic alloy Fe30Ni20Mn35Al15 (in at.%) was modified by cooling at different rates from 1623 K, i.e., above the eutectic temperature. The lamellar spacing decreased with increasing cooling rate, and in water-quenched specimens lamellae widths of similar to 100 nm were obtained. The orientation relationship between the fcc and B2 lamellae was found to be sensitive to the cooling rate. In a drop-cast alloy the Kurdjumov-Sachs orientation relationship dominated, whereas the orientation relationship in an arc-melted alloy with a faster cooling rate was fee((1) over bar 12)//B2(011); fee[1 (1) over bar1]//B2 [1 (1) over bar1] and fee(0 (1) over bar1); fee[011]//B2[(1) over bar(1) over bar0]. The hardness increased with microstructural refinement, obeying a Hall-Petch-type relationship. The strength of the alloy decreased significantly above 600 K due to softening of the B2 phase.