Kovar®, an austenitic Fe-29Ni-17Co alloy, was tested in monotonic tension and axial fatigue at stress ratios (minimum stress/maximum stress) of R= -1 and 0.1. Two annealed microstructures, having grain sizes of 14 and 65 μm, were tested. The strainhardening exponent of tensile flow curves gradually increased with strain, and then remained constant at a value of 0.23. Long-life (>108 cycles) tests indicated that fatigue limit behavior, well known for ferritic steels, also exists in Kovar. Fatigue limits (defned as the highest stress amplitude that did not cause failure in >107 cycles) were found to be 276 MPa (53% of UTS) for the 14-μm grain size and 207 MPa (41% of UTS) for the 65-μm grain size, both at R = -1. These two data points suggest that the fatigue strength of Kovar may have a strong grain size dependence. The fatigue strength of the 65-μm grain-size material was strongly affected by stress ratio. The R = 0.1 fatigue limit of this material was found to be 103 MPa (21% of UTS). Sharply notched (Kt = 6.8, 0.04-mm notch root radius) 65-μ,m grain-size specimens were tested at R = 0.1. Metallographic evidence of a small nonpropagating fatigue crack was found at the notch root of a specimen that was tested to >108 cycles. Fracture surfaces with well-defined striations were produced when fatigue crack-growth rates were high.