Sudden oak death (SOD), an emerging disease caused by the introduced oomycete Phytophthora ramorum, poses a serious threat to oak woodlands in the western US. Previous research has shown that foliar endophytes – fungi that live within apparently healthy leaves of plants without causing disease – can be effective as biological control agents in protecting trees against Phytophthora infection. Here, we examine the endophyte communities associated with foliage of seven oak taxa (Quercus arizonica, Q. emoryi, Q. gambelii, Q. grisea, Q. hypoleucoides, Q. rugosa, and Q. arizonica × Q. rugosa) in the Santa Rita Mountains in southeastern Arizona. In addition to providing a first description of the diverse community of endophytes associated with foliage of these healthy oaks, we compare this community with endophytes found in three other tree species in southern Arizona (Pinus ponderosa, Cupressus arizonica, and Platycladus orientalis). We show that (1) oaks in southeastern Arizona harbor endophytes that are characterized by high genotypic and phylogenetic diversity; (2) more closely related oaks do not necessarily share more endophytes than do more distantly related oaks; and (3) relative to endophytes of other trees in the same biogeographic region, oak endophytes are distinctive at both the genotype level and in terms of the major lineages of fungi they represent. These fungi, archived as living vouchers at the Robert L. Gilbertson Mycological Herbarium (ARIZ), represent a first step toward identifying the very poorly known communities of endophytes associated with plants in Arizona, and will provide the raw material for bioassays assessing endophyte-mediated biological control of Phytophthora spp.