The origin of the in-plane uniaxial magnetic anisotropy of epitaxial Fe(001) films on GaAs(001) has been controversially attributed either to a magnetoelastic interaction or to oriented interface bonds. In order to check the relevance of magnetoelastic contributions we performed magneto-optic Kerr effect (MOKE) and magnetoelastic stress measurements on Fe(001) and Fe₃₂Co₆₈(001) bcc epitaxial films on GaAs(001) in a thickness range from several monolayers (ML) up to 100 ML. The magnetoelastic coupling coefficient B₂ of 30 ML films is +(8.4±1.4) MJ/m³ for Fe(001) and -(23.2±3.7) MJ/m³ for Fe₃₂Co₆₈(001), respectively. In spite of the opposite sign of B₂ for Fe and Fe₃₂Co₆₈, the same sign of the uniaxial anisotropy constant K_U^eff is experimentally observed. It is concluded that the magnetoelastic interaction is not the dominating contribution to the uniaxial magnetic anisotropy in these systems.