Nine cultivars and four unnamed seedlings were assessed for their potency as parents in breeding for red core resistance. Field resistance was assessed by scoring the plants for survival on a site heavily infected with Phytophthora fragariae. As a generalisation it was found that the field resistance of the parents was closely related to that of their progenies. The information gained from analysis of data obtained from families raised in routine breeding was at least as useful as that obtained from a diallel cross or from a selfing experiment designed specifically to test the parents. Two unnamed Fragaria virginiana I derivatives (53Q13 and 60BQ8) and a Siletz derivative (60CE5) were the most resistant and proved to be the most potent sources of resistance, but their fruit quality was poor. Cambridge Favourite, despite its lack of vertical resistance, was at least as potent as Redgauntlet. General combining ability for red core resistance was more important than specific combining ability though the latter was evident with a few of the crosses.