Since 2007, a decline of young almond trees (Prunus dulcis) has been observed in different field-grown nurseries in Valencia (east-central Spain). Early symptoms in affected trees included chlorosis, wilting, cankers, and profuse stem gumming. A Phytophthora sp. was consistently isolated from cankers, roots, and soil of affected trees. It was a heterothallic species with amphi- gynous and/or paragynous antheridia, and its morphological features did not conform to any of the described Phytophthora species. Pathogenicity was proved by artificial inoculation, completing Koch's postulates. All isolates were sensitive to the phenylamide fungicides metalaxyl and mefenoxam. Amplification and sequencing of the internal transcribed spacer (ITS) region, translation elongation factor 1 alpha gene (EF-1a), the tubulin (fS-tub) gene, and the region containing the mitochondrial cytochrome c oxidase subunit 1 (coxl) gene fragment identified the species as Phytophthora taxon "niederhauserii" , and phylogenetic analyses placed it in Phytophthora Clade 7b. © 2010 The American Phytopathological Society.