A study was conducted into the structural features of martensitic transformation in four Iron—arsenic alloys, ranging between 0.22 and 1.68% As. The structures produced by quenching these alloys from a suitable austenitizing temperature into iced brine were studied systematically by optical and electron microscopy. The martensitic substructural units were lath shaped. The orientation relationships between adjacent laths were determined using selected-area electron diffraction techniques, and the observed orientation relationships were consistent with the theory that adjacent laths adopt different variants of the Kurdjumov-Sachs orientation relationship. Single surface trace analyses established that the laths have a habit plane close to {1 1 0}α and a long direction parallel to 〈1 1 1〉α. The results are compared with the prediction of recent phenomenological theories of martensitic transformation.