A total of 6830 northern red-legged frog Rana aurora aurora tadpoles were examined under a dissecting microscope for oral disc, integumentary, and cloacal abnormalities in 13 ponds in and near Redwood National Park in northern California. Of these, 163 tadpoles were collected for histopathological investigation, including 115 randomly collected individuals, 38 collected with oral disc abnormalities, and 10 collected due to severe morbidity of unknown etiology. The tadpoles were infected with 8 parasites, including Batrachochytrium dendrobatidis (the amphibian chytrid), trematodes, leeches, and protozoa. Chytridiomycosis was detected at an overall prevalence of 6.4%, but prevalence was higher in tadpoles with oral disc lesions than in those with normal oral discs (43.5% versus 6.1%). Interestingly, infection was associated with some environmental and co-infection risk factors. Individual tadpoles possessed 0 to 5 species of parasites in varying intensities. Apiosoma sp. was the most prevalent (66%) and widespread. Tadpoles infected with B. dendrobatidis had a lower diversity of oral parasites than those uninfected. During the field portion of the study, a large number (approximately 500) of moribund and dead tadpoles was seen occurring at multiple locations within and surrounding Redwood National Park. Ten animals were collected for histological examination and a diverse protozoal infection was discovered, including some known pathogens of fish. This study is the first reporting parasitism and disease in natural populations of northern red-legged frogs.