Aphelids (Aphelida) are poorly known parasitoids of algae that have raised considerable interest because of their phylogenetic position as phagotrophic protists sister to Fungi. Together with Rozellida and Microsporidia they have been classified in the Opisthosporidia but seem to be more closely related to the Fungi rather than to the Cryptomycota and Microsporidia, the other members of the Opisthosporidia. Molecular environmental studies have revealed high genetic diversity within the aphelids, but only four genera have been described: Aphelidium, Amoeboaphelidium, Paraphelidium and Pseudaphelidium. Here, we describe the life cycle of a new species of Aphelidium, Aph. arduennense. Molecular phylogenetic analysis of its 18S rRNA indicates that Aph. arduennense is sister to Aph. tribonematis, and together with Aph. melosirae they form a monophyletic cluster. Within the aphelids, this cluster is distantly related to Paraphelidium and Amoeboaphelidium.