A new alder (Alnus) disease caused by a fungus of the genus Phytophthora was identified for the first time in the United Kingdom in 1993. A project aiming at establishing a survey of the situation regarding this disease in Belgium is in progress in our laboratory. The fungus was isolated on affected trees in different sites subject to flooding from adjacent rivers in the South of Belgium. About 10 "Phytophthora alni" were identified by colony characteristics and gametangial morphology. Attempts to isolate the fungus from soil sample have been carried out using a baiting technique. Studies are also performed in order to determine the parameters that contribute to the disease development and to propose recommendations on disease management and control.