Abstract: Oak decline has been a serious problem in Mediterranean countries since the beginning of the twentieth century. The invasive pathogen P. cinnamomi has been associated with intense mortality of oak trees in the Iberian Peninsula, southern France and Italy. Although oak decline has been reported also in North Africa, P. cinnamomi was never recovered in any of the surveyed stands, and tree decline has been associated with other diseases, pests, climate and human intervention. The aim of this work was to investigate the occurrence of Phytophthora species and their possible involvement in the decline of cork oak trees in Algeria. A declining cork oak forest located along the Mediterranean northwest coast was surveyed between 2015-2016. Symptomatic trees showed thinning and crown dieback, withering of leaves and death. Root rot and extensive loss of both lateral small woody roots and fine roots were observed. To isolate Phytophthora, roots and soil samples collected from 18 declining and 8 asymptomatic cork oak trees were baited using fresh cork oak leaves. Isolations were made on SMA, a selective medium for Phytophthora spp. Isolates were identified using morphological analysis and DNA-based techniques. Phytophtora cinnamomi was the most frequent species isolated and three other Phytophthora species identified as P. gonapodyides, P. multivora and P. quercina were detected. Comparative aggressiveness of these Phytophthora species was assessed on 3-month-old seedlings of Quercus afares, Q. canariensis, Q. ilex and Q. suber. This is the first study looking at the diversity of Phytophthora pathogens in Algerian oak forests.