In order to investigate the fatigue behavior during irradiation of the EUROFER 97 steel, similar to that which would occur in a fusion reactor, a series of six in situ experiments was conducted using a specially designed proton irradiation system. All tests were performed at a total strain range of 0.8%, at 150 and 250 degrees C. In two experiments, to check the influence of the dislocation structure, the specimens were first fatigued prior to the irradiation and then only fatigued under beam. In one case, when the specimen reached the predicted number of cycles to failure, the beam was turned off and the test continued without a beam. Unexpectedly, the specimen recovered and attained almost the same number of cycles as without the beam irradiation. Secondary experiments were conducted for measuring the irradiation hardening under static stresses, as well as for measuring the flow stress under irradiation. (C) 2007 Elsevier B.V. All rights reserved.