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Load-controlled fatigue tests were conducted for four positive R values ona low-alloy TRIP steel for two different heat treatments: an optimal treatment leading toretained austenite (next to ferrite, bainite and martensite) and a non-optimal treatmentleading to a ferritic-martensitic steel. A significantly increased resistance to fatiguecrack growth was found for the optimal case relative to the non-optimal case. Theamount of crack closure was found to be larger in case of the non-optimally treated(ferritic-martensitic) steel. Martensite transformation, as was observed on the basis ofan increasing hardness during straining in static tensile tests, was only found to occurwithin the monotonic plastic zone formed during fatigue.