Mutualistic interactions between species are receiving increased attention from ecologists, although research lags far behind analogous work on competition or predator-prey interactions. Most research has focused on rather showy mutualisms such as pollination or fruit dispersal and has suggested that mutualisms are more important in tropical communities than in temperate communities (67). Plant-microbial mutualisms, in contrast, have prompted little ecological research. Plant-microbial associations are more difficult to observe and manipulate than plant-animal associations. Many plants are always infected (e.g. legumes by rhizobia, forest trees by mycorrhizal fungi), so it is easy to consider the microorganisms merely as a special type of plant organ. Further, plant-microbial mutualisms historically have been outside the realm of ecology, in other areas of biology like microbiology and mycology. Recent research has revealed a widespread mutualistic association between grasses, our most familiar and important plant family, and endophytic fungi. Asymptomatic, systemic fungi that occur intercellularly within the leaves, stems, and reproductive organs of grasses have dramatic effects on the physiology, ecology, and reproductive biology of host plants. Through the production of toxic alkaloids, endophytic fungi defend their host plants against a wide range of insect and mammalian herbivores. Poisoning of domestic livestock has spurred a great deal of research on endophytic fungi in pasture grasses. This research has shown clearly that plants benefit from