The magnetic properties of uncovered Fe/ZnSe/GaAs(1 0 0) ultrathin films have been determined in situ by magneto-optical Kerr effect (MOKE). Fe films up to 10 monolayers (ML) thick were deposited on c(2 x 2) Zn-rich ZnSe/GaAs(0 0 1) surfaces at 180 degrees C. We have studied the thickness dependence of the in-plane lattice parameter of the Fe films and of the MOKE hysteresis loops in the longitudinal geometry, at 150 K, under magnetic fields up to 0.1T applied along the [1 1 0] and [1-1 0] directions of the ZnSe(0 0 1). Reflection high energy electron diffraction show that in the low thickness regime the Fe films present an in-plane structural anisotropy characterized by an expansion along the [1 1 0] direction. Hysteretic loops were obtained only starting from similar to 5 ML Fe. We found the onset of an uniaxial magnetic anisotropy with [1 1 0] magnetic easy axis at 7 ML Fe. (C) 2005 Elsevier B.V. All rights reserved.