Chytridiomycosis, a disease caused by the chytrid fungus, Batrachochytrium dendrobatidis, has been reported recently in amphibian populations throughout the world, and has been associated with many cases of population declines and extinctions.  In some areas of the Sierra Nevada of California the disease appears to be the causal factor in the rapid extinction of local populations of the mountain yellow-legged frog, Rana muscosa, within a few years of the first detection of the fungus.  In other areas, however, R. muscosa populations appear to persist for many years, despite high levels of infection in tadpoles. R. muscosa has a long-lived tadpole stage (1-4 years), which can become infected with the fungus but is not affected by the disease until metamorphosis. I will present the results of a model that illustrates the importance of within-host pathogen dynamics, and suggests that the tadpole stage plays a key role in the long-term persistence of the pathogen in R. muscosa populations.