Abstract Duplex specimens were used to measure the crack arrest toughness ( K Ia ) of a nodular cast iron. Over the temperature range from −40 to +23°C, K Ia was close to published values of the dynamic initiation toughness ( K Id ) of nodular irons with similar microstructures. However, K Ia appeared to be somewhat more temperature dependent than K Id . The temperature dependence of K Ia is also somewhat greater than would be consistent with dependence of K Ia is also somewhat greater than would be consistent with fractographic observations, which show almost no variation in fracture appearance over the temperature range investigated. When referred to the ductile/brittle transition temperature, the nodular iron K Ia values are similar to those of reactor-pressure-vessel steels. A detailed comparison of the raw data with previous results on ferritic steels, combined with finite element analysis, suggests that the crack-arrest-toughness values reported here are either accurate or slightly conservative.