A field survey of soybean aphid, Aphis glycines Matsumura, and its natural enemies was conducted during summer 2002 in Langfang, northern China (116.4° E, 39.3° N). Aphids colonized soybean when plants were still small in early July. After a lag of 2 wk, aphid density increased rapidly in late July, reaching a peak of 114 ± 46 aphids per five soybean plants on 1 August. The population declined to a plateau immediately after this peak and then declined again starting in mid-August, although a second small peak occurred in late August. The finite rate of increase varied from zero- to five-fold, and the aphid seemed to be limited by natural enemies. The main species of natural enemy were the aphid parasitoid Lysiphlebus sp., the aphid predators Propylaea japonica (Thunberg), Scymnus (Neopullus) babai Sasaji, and Paragus tibialis (Fallén). In a field exclosure experiment, A. glycines density in small-mesh cages peaked three-fold higher than in large-mesh cages and 12-fold higher than on uncaged plants, indicating that natural enemies did indeed limit aphid density.