Aphelids are a poorly known group of algal parasites that have raised considerable interest because of their pivotal phylogenetic position as a sister lineage to the kingdom Fungi. Recent breakthroughs in sampling, sequencing, and bioinformatical analyses of environmental nucleic acids have revealed a magnificent biodiversity of aphelids. Nevertheless, only four genera have been described, Aphelidium, Paraphelidium, Amoeboaphelidium, and Pseudaphelidium. Among these, 18S rRNA gene sequences are available for all except for the marine genus Pseudaphelidium. Most of the environmental nucleic acid data is from analyses of freshwater samples. We isolated two new marine aphelid strains, X-138 and X-139, and herein describe the life cycle of Protaphelidium rhizoclonii gen. et sp. nov., a parasite of the green alga Rhizoclonium sp., and provide the first 18S rRNA gene sequences for cultivated marine aphelids. The new marine aphelid life cycle is mostly typical for aphelids but also includes previously undescribed stages. Molecular phylogenetic analysis indicated that Protaphelidium rhizoclonii is a member of an environmental clade at the base of the aphelid tree. © German Mycological Society and Springer-Verlag GmbH Germany, part of Springer Nature 2024.