Some parasites modify characteristics of their arthropod intermediate hosts to facilitate transmission to the terminal host. Here we show that a tetradonematid nematode parasitoid alters the behavior and morphology of its host ant, Cephalotes atratus, in ways that likely promote its dispersal to new ant colonies by frugivorous birds. From mid-May to early July, approximately 5% of the foraging workers of an infected C. atratus colony in Panama have bright red gasters, which are full of nematode eggs. Uninfected ants are completely black. Infected ants are 12% smaller and 41% heavier than their uninfected sisters. While foraging, infected ants maintain their gasters in an elevated position, dramatically increasing their apparency. Gasters of infected workers readily detach from the body at the postpetiole-gaster junction. The average tensile force required to break the junction was an order of magnitude less for infected ants (18g) than for normal ants (254g), and was significantly less than that required to dislodge an ant from natural substrate while in a defensive posture (47g). Preliminary experiments with tethered ants and clay models suggest that infected ants are preyed upon more often, and that frugivorous birds may confuse the red gasters for small red fruits (e.g., Psychotria spp.) appearing in the forest at the same time.