A pyrethrin drench appeared to be as efficient as or more efficient than soil sifting as a method of detecting adult alfalfa weevils,Hypera postica(Gyllenhal), in the soil. Adult weevils fed on cut alfalfa treated with32P were labeled and could be detected in the field for at least 6 wk. The procedures were used to determine that most adult weevils left the study field during the month following 1st harvest for more sheltered locations, but they were not found congregated in any single location.