Abstract: Five cultivars of alfalfa (Medicago sativa) were used to identify their antibiosis to two colour morphs of the pea aphid Acyrthosiphon pisum. Life tables were constructed at a constant temperature of 21-23℃, relative humidity of 50%-70%, and illumination time of 14 h per day. The antibiosis of the same alfalfa and of different alfalfa cultivars to the two colour morphs differed. Using the intrinsic rate of increase (R_m) as the index of alfalfa resistance to the aphid, the resistance of Russian Medicago varia (RM) to green aphid was stronger than to the pink aphid but the resistance of Algonquin (AL) to the pink aphid was stronger than to the green aphid, while G₅ showed a certain resistance to both colour morphs. The order of antibiosis of the different alfalfa cultivars to pink aphid was RM<SW<AL=G₃<G₅, and to the green aphid was AL<RM<SW= G₃<G₅.