Few experimental studies have addressed the demographic consequences of plant—microbe symbiosis in natural communities. In this study the survival, growth, and reproduction of tall fescue grass (Festuca arundinacea), smut grass (Sporobolus poiretii), and green sedge (Cyperus virens) infected by the imperfect fungus Acremonium coenophialum, the ascomycetous fungus Balansia epichloe, and B. cyperi, respectively, were compared with uninfected conspecifics in field experiments conducted in natural plant communities in southern Louisiana. Equal numbers of infected and uninfected ramets of each species were propagated in the greenhouse from infected and uninfected seed lots (F. arundinacea) or from field—collected infected and uninfected plants (S. poiretii and C. virens). Ramets of the two grasses were planted into a grassland community where the two species were dominant members of the community. Ramets of the sedge were planted into a marshy area where C. virens occurred naturally. Numbers of tillers were counted for each ramet upon planting for subsequent use as a covariate, and each ramet was censused from one to three times a year for three consecutive years. All survivors were then harvested and aboveground dry biomass was determined. For F. arundinacea, infected ramets exhibited higher survival, flowering frequency, vegetative tiller production, and biomass over the 3—yr period. Relative fitness, calculated from survival and inflorescence production, was nearly twice as high for infected ramets, indicating a mutualistic relationship between F. arundinacea and its endophytic fungus. For S. poiretii, growth and survival of infected and uninfected ramets were similar, but flowering was greatly suppressed in infected ramets. Relative fitness of infected ramets was only °1/10 that of uninfected ramets, indicating a pathogenic relationship. There was evidence of substantial contagious spread of the fungus in the experimental population. Infected ramets of C. virens had significantly higher survival over 3 yr and tended to be bigger than uninfected ramets. Inflorescences were produced by both types of ramets, but the inflorescences of infected ramets were always aborted. The results indicate that, within a closely related group of fungi, both pathogenic and mutualistic relationships with host plants exist. Maternally inherited, seed—borne endophytes similar to the one in F. arundinacea are found in any grasses, suggesting that analogous mutualisms are common. The greater survival and vegetative vigor of infected C. virens call into question the traditional notion of fitness as applied to plants capable of extensive asexual reproduction. The biological mechanisms responsible for the greater survival and growth of infected plants probably include induced alterations in the host plant's hormonal balance, greater tolerance to abiotic stresses, and increased resistance to herbivores and pathogens.