During extensive surveys for Phytophthora associated with alder mortality in western Spain, isolates were consistently recovered from necrotic bark of the collar and lower stem of Alnus glutinosa. Morphological and molecular characteristics of the selected isolates together with their physiology were examined. Morphologically, the isolates were homothallic and characterized by abundant production of terminal oogonia predominantly with two-celled amphigynous antheridia exhibiting ornamentation. Simple sporangiophores were observed bearing terminal, non-papillate, ellipsoid to ovoid sporangia. Colony growth patterns developed on V8 juice agar (V8A), carrot agar (CA), corn-meal agar (CMA) and potato-dextrose agar (PDA) showed uniform, radial or irregular growth patterns with appressed and/or woolly morphology. Temperature (2 °C, 5 °C, 10 °C, 15 °C, 20 °C, 25 °C, 27 °C, 30 °C and 32 °C), pH (5, 7, 9 and 11) and osmotic potential (−0.6, −1.2, −1.8 and −2.4MPa) significantly influenced radialmycelial growth of the isolates under laboratory conditions. Molecular analyses including ITS DNA region sequencing and subspecies specific primers amplification confirmed identification of the isolates as Phytophthora alni ssp. alni. Further research is under way to carry out more surveys in order to determine the extent of damage and severity of the disease in different regions of Spain.