Egg populations of the Pea Aphid, Acyrthosiphon pisum (Harris), overwintering on lucerne and hop trefoil have been followed. On hop trefoil the overwintering mortality of these eggs was 83 per cent. Although the mortality was apparently less on lucerne, a wide discrepancy existed between the two crops in the ratio between the total numbers of eggs at the beginning of eclosion and the number of Aphids which successfully established themselves the following spring. The peak number of fundatrices on lucerne was three times the egg total at the beginning of eclosion whereas on hop trefoil it was only half. Many more eggs than were recorded were undoubtedly laid on the lucerne plants and the high number of fundatrices subsequently found must have emerged from eggs that had fallen or been knocked off the host-plant.