Pea aphid (Acyrthosiphon pisum Harris) is a pest of lucerne (Medicago sativa L.) in various countries of Europe and North and South America. The objective of this study was to determine the inheritance of pea aphid resistance in lucerne using a 6-parent diallel crossing design. Three parents (‘CUF101’, ‘Monarca’, the experimental cultivar Milfeuil-P4) were known to be resistant, two were susceptible (‘Europe’ and ‘Diane’) and one was intermediate (‘Cinna’). Resistance was measured using a no-choice laboratory screening technique on lucerne seedlings. Two aphid infestations were made, and stopped 13 days after the first infestation. The number of surviving seedlings and the dry weight of surviving seedlings on day 26 after the first infestation were recorded. General combining ability (GCA) was significant and much more important than specific combining ability (SCA). There was a strong correlation between the resistance level of the cultivars and their GCA. The hypothesis of a monogenic inheritance could not be confirmed. These results suggest a multigene inheritance for pea aphid resistance in lucerne. Inheritance of pea aphid resistance in lucerne is mainly additive, therefore rapid genetic improvement can be expected using recurrent selection.