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For the Mediterranean Sea, and indeed most of the world's oceans, the biodiversity and biogeography of eukaryotic pathogens infecting marine macroalgae remains poorly known, yet their ecological impact is probably significant. Based on 2 sampling campaigns on the Greek island of Lesvos in 2009 and 1 in northern Greece in 2012, this study provides first records of 3 intracellular eukaryotic pathogens infecting filamentous brown algae at these locations: Eurychasma dicksonii, Anisolpidium sphacellarum, and A. ectocarpii. Field and microscopic observations of the 3 pathogens are complemented by the first E. dicksonii large subunit ribosomal RNA (LSU rRNA) gene sequence analyses of isolates from Lesvos and other parts of the world. The latter highlights the monophyly of E. dicksonii worldwide and confirms the basal position of this pathogen within the oomycete lineage (Peronosporomycotina). The results of this study strongly support the notion that the geographic distribution of the relatively few eukaryotic seaweed pathogens is probably much larger than previously thought and that many of the world's marine bioregions remain seriously undersampled and understudied in this respect.