The alfalfa weevil, Hypera postica(Gyllenhal) (Coleoptera: Curculionidae), is one of the important pests of milk vetch, Astragalous sinicus L.(Fabaceae), in Japan. To obtain basic data for establishing control measures against the weevil, we surveyed adult behavior of the weevil during the period from the time of emergence in milk vetch fields to the time of recolonization of the fields after aedtivation. The density of emergent adults varied with fields in May. Most adults were aestivating mainly under the bark of trees and between the bark and materials covering trees. Some were collected by sweeping grassy places and branches of trees. The adults started to leave the places used for aestivation in late September, and seemed to spend about a month somewhere on their way to the field. In late October, they came back to the milk vetch fields. The number of recolonized aadults did not vary with the field at the beginning but varied greater in the late season. From these observation, we concluded that control measures could be effective when applied to the adults moving from the places used for aestivation to the milk vetch fields.