Male apple maggot flies, Rhagoletis pomonella (Walsh), were present on ceresin wax domes (the site of oviposition and the site of assembly for mating) in much greater relative frequency when caged with 1 or more females or with several other males than when caged alone. Field and laboratory experiments subsequently revealed that a principal stimulus eliciting this greater degree of male assembly was an unidentified chemical(s) deposited on the surface of apples and ceresin domes by both sexes (but in greater amount by mature females) causing arriving males to spend up to twice as much time there as on untreated fruit. The presence of the chemical did not elicit any greater attraction of males to the fruit. The consequence of the longer male residency on domes with the chemical was male aggregation and an ensuant higher frequency of mutual encounter and attempted copulation than on untreated domes. While the chemical acts to arrest males, we theorize that mere detection of its presence may be sufficient to cause males to remain in the vicinity and watch for females.