Cowpea aphid (Aphis craccivora Koch) infestation accumulated phytohormone salicylic acid (SA) in leaves of soybean (Glycine max cv. Namdan) as these soybean plants were at stages R1 (beginning to bloom) and R3 (beginning pod development). Activity of major enzymes involving biosynthesis of SA, such as phenylalanine ammonia-lyase (PAL), benzoic acid 2-hydroxylase (BA2H), was also enhanced under aphid effect. Changes in activity of these enzymes was closely correlated with the content of SA in the tissues. The enhancement of the SA biosynthetic pathway may reduce the effects of A. craccivora on G. max cv. Namdan plants at reproductive growth stages