Ternary alloys of Fe 65 (Ni1-x Mnx)35 have an f.c.c. structure over a whole range of composition and show a transition from ferromagnetic to antiferromagnetic at around the composition of × =0.3. In this transition region it is quite interesting to study their magnetic properties because of the possible coexistence of ferromagnetic and antiferromagnetic regions. This paper mainly deals with the exchange anisotropy of these alloys at low temperatures down to 1.4°K obtained by measurements of rotational hysteresis loss and unidirectional anisotropy. The results show an evidence for the coexistence of ferro- and antiferromagnetic regions at low temperatures. A temperature TK, where the rotational loss disappears due to the small anisotropy constant of antiferromagnetic regions, is considered to indicate a lower limit of the Néel temperature, TN of the regions. Values of TK for x ≤0.4 connect smoothly onto values of TN for x ≥0.4. © 1967, THE PHYSICAL SOCIETY OF JAPAN. All rights reserved.