Aphid occurrence, population dynamics, and sampling procedures were studied in three consecutive broccoli plantings in southern California. Myzus persicae (Sulzer) was dominant during the fall and spring, and Brevicoryne brassicae (L) was prevalent in winter. B. brassicae was the primary aphid contaminant of broccoli heads in all crops. Hyadaphis erysimi (Kaltenbach) and Macrosiphum euphorbiae (Thomas) occurred at low population levels and were of minor importance. Aphid distribution was aggregated when densities were low [log (x + 1) <1.0 per plant] but became random as populations increased. Sampling procedures based on 10-plant samples were useful in estimating aphid population when densities exceeded a log (x + 1) value of 0.65 per plant. Aphid parasitization did not exceed 6% of available hosts in any planting and was not a major mortality factor.