The role of large prior austenite grain sizes in promoting low ductility of fully pearlitic eutectoid steels has been investigated. Independent variation of the pearlite interlamellar spacing,S p, and the prior austenite grain size enabled determination of the microstructural feature controlling ductility in these steels. Tests on a variety of specimens over the temperature range —196 °C to 310 °C showed that specimens heat treated to contain a large prior austenite grain size consistently exhibited lower fracture strains (i.e., ductility), regardless of the type of fracture present, than did fine grained specimens. It appears that the prior austenite grain size controls ductility through its influence prior to catastrophic failure.