Despite their abundance in the field, and their suspected role in regulating the abundance of their host population, hyphochytrid pathogens of brown algae have been hardly studied. Using laboratory cultures, we document here the life cycle of Anisolpidium ectocarpii, a pathogen of Ectocarpus and other filamentous brown algae, and present preliminary observations on Anisolpidium rosenvingei. Consistent with earlier reports, the zoospores of both species have a single anterior flagellum, which justified the placement of Anisolpidium amongst the Hyphochytridiales (Hyphochytridiomycota). Unexpectedly, nuclear (SSU rRNA) and mitochondrial (cox 1, cox2) markers regroup A. rosenvingei, A. ectocarpii and other marine environmental sequences into a hitherto unrecognised monophyletic clade within the oomycetes (Oomycota), most closely related to the Olpidiopsidiales and Haliphthorales. The Anisolpidium genus is therefore entirely distinct from the Hyphochytridiales and represents the first unquestionable instance of an anteriorly uniciliate oomycete. We also show that A. ectocarpii can complete its infection cycle in a broad selection of species from various brown algal orders, suggesting that species delimitation within the genus Anisolpidium should not merely be based on the identity of the algal host, as is presently the case. Finally, a working hypothesis is generated in an attempt to establish a new criterion for the separation of hyphochytrids from oomycetes, based on the point of zoospore cleavage.