A 6-parent diallel crossing design was performed to analyse the genetic inheritance of the resistance of alfalfa seedlings to the pea aphid. Resistance was measured using no-choice laboratory screening techniques. Two aphid infestations were made and stopped 13 days after the first infestation. The dry weight of surviving seedlings was recorded 26 days after the first infestation. Three parents (CUF101, Monarca, the experimental cultivar Milfeuil-P4) were previously tested as resistant, two were susceptible (Europe and Diane) and one was intermediate (Cinna). General combining ability (GCA) was significant and much more important than specific combining ability (SCA). These results indicated that breeding for pea aphid resistance in alfalfa should produce a high genetic progress.