In most structural steels, the critical plane-strain stress-intensity factor, KIc, which represents the inherent ability of the material to resist crack propagation, increases with increasing test temperature. In many steels, the rate of increase of KIc with test temperature increases markedly above a temperature region which is designated the fracture-toughness transition region. Because of this transition behavior with temperature and the inherently high fracture toughness of many steels, very thick specimens must be tested to determine valid KIc values. However, the large size of these specimens and the cost of conducting the tests minimize the usefulness of this procedure as a research tool for analyzing the fracture behavior of steels under plane-strain conditions. Therefore, as part of a long-range program to obtain KIc values from small specimens, the relationship between KIc and the ductility measured in a small plane-strain tension test was investigated. The results showed that the temperature transition of the plane-strain tensile ductility at fracture, εf.ps, paralleled that of the KIc, whereas the temperature transition of the axisymmetric ductility (measured in a conventional round tension specimen) was quite different from that of the KIc. At any temperature, the relationship between these properties for steels ranging in yield strength, σy, from 80 to 250 ksi is approximated by the equation KIc=Aσyε2f.ps where A is a constant for a given steel. This relationship indicates that for certain steels the KIc value approaches an upper limit or shelf value with increasing temperature, because as the test temperature increases, the plane-strain tensile ductility approaches an upper limit and the decrease in yield strength becomes negligible. The existence of a KIc shelf would imply that in thick sections the material could fail suddenly, even at elevated temperatures. However, because of the rapid rate of increase of the plastic-zone size with temperature (and thus the through-thickness plastic flow at the crack lip), steels in conventional thicknesses would lose plane-strain constraint at temperatures approaching that for the KIc shelf, and therefore only ductile behavior would be expected at shelf temperatures.