Epiphytic plants have evolved a diverse array of associations with ants that promote the provisioning of moisture, nutrients, or physical protection (Davidson & Epstein 1989). Ant-epiphyte associations are often based on: (1) the occasional use of carton (= paper) nests constructed by ants as substrate for establishment and growth of epiphytes (Longino 1986); (2) the production of extrafloral nectar by epiphytes to attract opportunistic ant 'guards' (Bentley 1977, Fisher & Zimmerman 1988); (3) the production of'domatia' or hollow cavities in epiphytes that house ant associates (Benzing 1970, Huxley 1978); or (4) the seed dispersal of epiphytes by ants, which create 'ant-gardens' (Davidson 1988). These associations are not exclusive; they can be found in combination and may result from opportunistic or obligate interactions between ants and epiphytes. In epiphytic orchids, extrafloral nectaries are the most common structures that give rise to associations with ants (Fisher & Zimmerman 1988). Extrafloral nectaries may be efective in providing a facultative means of defence because arboreal ants are the most common group of insects in the tropical forest canopy (Benson 1985, Erwin 1983, Wilson 1987). A few orchid species have been reported to occur only on ant nests (e.g. Coryanthes, Ep)idendruin irnatopkyllurn Lindl.) but no detailed studies have investigated these associations (Dressler 1981). Two genera of orchids (Caldarthiron and Schonburgkil section Chlzazno-Schomburgkia) are known to provide nesting sites for ants in hollow pseudobulbs (Dressler 1981, Fisher & Zimmerman 1988, Kennedy 1979, Rico-Gray et al. 1989). Although a few studies have demonstrated positive effects of obligate, symbiotic ant associates on host plants (Janzen 1972, Schupp 1986), the effect of facultative, symbiotic ants on host plants has received less attention (Beattie 1985, RicoGray 1987). This investigation experimentally evaluates the effect of facultative ant occupants on the production of reproductive structures in an epiphytic orchid, Calart/ithr-oti bilamellatutm (Rchg.f.) Schult., in Panama. The study was conducted during the dry season of 1987 at the Smithsonian