Field tests showed that during the first 2 weeks of August more sexually mature apple maggot flies,Rhagoletis pomonella(Walsh), were captured in apple trees containing real fruit than in trees without fruit or trees containing artificial apples. During the last 2 weeks of August, when the numbers of flies were rapidly declining, similar numbers of flies were captured in all 3 groups of trees. Only a small number of fruit in a mature apple tree was needed to increase the capture of flies, but an equal number of artificial apples had no such effect. This demonstrated that the maturity of apples affects the response of the flies, and suggests that factors other than visual stimuli are involved.