The research on the occurrence of herbivorous heteroptera of Lygus genus on alfalfa (Medicago sativa L.) was carried out over 2005-2006 in the Hamedan province (Western Iran). The analysis of the faunistic material collected from Hemiptera showed the occurrence of 4 species where dominant Heteroptera were represented by: Lygus rugulipennis Poppius, L. pratensis L., L. gemellatus H.-S. and L. punctatus Z. The maximum abundance of these species coincided with full flowering alfalfa. Among Lygus species, L. rugulipennis P. was the most highly abundant in alfalfa's the main species in these fields.