Summary: Leaf-cutting ants in the genus Acromyrmex are obligately dependent upon a mutualistic fungus as their primary food source. One mechanism employed by fungus-growing ants to control general infections of the fungus garden is through secretions of the metapleural glands, which produce a broad spectrum of antibiotics. In addition, the ants carry a mutualistic filamentous bacterium (an actinomycete) on their cuticle, which produces antibiotics that suppress the growth of Escovopsis, a specialised parasite of the gardens of fungus-growing ants (Currie et al., 1999a). We show that a clear trade-off between these defence mechanisms exists at the level of individual workers. Major workers have relatively small metapleural glands, proportional to body size, have a high abundance of the mutualistic bacterium, and are, when carrying the bacterium, most abundant in the lower part of the fungus garden, where Escovopsis, if present, is also most abundant. Minor workers have relatively large metapleural glands, proportional to body size, but have a relatively low abundance of the bacterium and are most abundant at the top of the fungus garden, where a variety of potentially infectious microbes enter the colony on the substrate used to grow the fungal mutualist. The two sympatric species investigated, Acromyrmex octospinosus and A. echinatior, have quantitatively different combinations of these defence mechanisms, suggesting that the optimal investment in alternative defence mechanisms in different ant species depends on differences in caste allocation or parasite pressure.