Abstract: Oak forests are far the most important ecosystems in the Mediterranean regions. However, their sustainability is under threat due to severe decline process, climate change and lack of natural regeneration. The oomycetes Phytophthora spp. have been associated with oak decline, with P. cinnamomi being the most widespread species. Besides killing adult trees, Phytophthora can in turn act as damping-off pathogen affecting the natural regeneration. This study aimed to explore the diversity of Phytophthora species occurring in Sardinian oak forests (Italy) and to investigate the variation in early survival of oak seedlings to Phytophthora infections. Soil and root samples underneath oak trees and from seedlings showing symptoms of Phytophthora infection were baited using fresh oak leaves. Several Phytophthora species were isolated and identified based on morphology and DNA sequence analyses. The susceptibility of germinated acorns to Phytophthora infections was tested by immerging growing taproot in a zoospore suspension. Although at different rates, Phytophthora species were able to cause a significant reduction of root development. In the field, the lack of seedling recruitment was assessed in 1 m2 plots selected in Phytophthora infested and disease-free sites. The results of this study provide new insights in the aetiology of oak decline and lack of seedling recruitment. The ecological implications of these findings are discussed.