Although parasitism of ants (Hymenoptera: Formicidae) by phorid flies (Diptera: Phoridae) has been reported by a number of investigators, their behavior is still poorly understood (Pergande, 1901;Wasmann, 1918; Borgmeier, 1925 Borgmeier, , 1931 Borgmeier, , 1963 Borgmeier, , 1969 Donisthorpe, 1927;Steyskal, 1944). Recently Williams et al. (1973), Williams and Whitcomb (1974) In one particular case, we observed a major worker of S. invicta under continuous attack by a lone phorid. We collected both host and parasite, placed them in a 20 ml vial. Once in the vial, the phorid attempted repeatedly to land on the agitated worker which was running back and forth in the vial trying to elude the fly. After approximately 5 min, the phorid landed successfully on the posterior end of the hosts' head capsule, deposited one oval egg, and then began searching for an exit from the vial, paying no attention to the ant. The ant, however, attempted to remove the egg with its antennae and forelegs. However, the placement of the egg at the base of the head capsule near its junction with the thorax made it impossible for the ant to do so. After several minutes of persistent attempts to dislodge the egg, the parasitized ant ceased grooming activities. We do not know whether or not the egg would have been removed by nestmates had the worker returned to its nest. Replicates (n-5) of the above procedure were conducted using newly-mated female sexuals and other P. obtusus. These P. obtusus displayed no visible attack or oviposition behaviors while enclosed in a vial with individual S. invicta queens.During this study, five other species of ants were observed foraging for baits in the study area. Feener (1981) noted that phorids of the genus Apocephalus attack major workers of the ant Pheidole dentata and "stampede" the ant, thus overcoming its defensive behavior. P. obtusus appeared in our study to promote a similar response from its host. Parasitism of P. dentata also affected the competitive balance between it and S. texana (Feener, 1981 promoting the caste-bias in host selection observed in this study may be that major workers provide more nutrition for developing parasite larvae than non-major workers. This would be especially the case if P. obtusus larvae develop only in the head, a pattern observed in other species of ant-specific phorids (Pergande, 1901).Finally, major workers may represent a more reliable resource.They are the longest-lived of the worker castes (Porter and Tschinkel, 1985), and their relatively reduced aggressiveness renders them least likely of all workers to incur injury or death from fighting. Therefore we suggest that the probability of an individual host surviving the requisite period of parasite development in that host is, on average, greater for major workers than for other workers. If so, selection would favor phorids that parasitize major workers.ACKNOWLEDGEMENT