Occurrence of the bird cherry-oat aphid(Rhopalosiphum padi L.) on bird cherry (Prunus padus L.) was studied in Eastern Poland. First fundatrix of R. padi on P. padus were observed in the first decade of April. The bird cherry-oat aphid population developed on the primary host during May. Its maximum reached the peak density until the middle of May, and in the third decade of May numerous alatae were formed and spring migration of the aphid onto the summer hosts started.