Aphelids areapoorlyknowngroupofparasitoidsofalgaethathaveraisedconsiderableinterestdueto their pivotalphylogeneticposition.TogetherwithCryptomycotaandthehighlyderivedMicrosporidia, theyhavebeenrecentlyre-classifiedasOpisthosporidia,beingthesistergrouptofungi.Despitetheir hugediversity,asrevealed by molecular environmentalstudies,andtheirphylogeneticinterest,only three generahavebeendescribed(Aphelidium, Amoeboaphelidium, and Pseudaphelidium), fromwhich 18S rRNAgenesequencesexistonly for Amoeboaphelidium species. Here,wedescribethelifecycle and ultrastructureof Aphelidium aff. melosirae, andprovidethefirst18SrRNAgenesequenceobtained for this genus.Molecularphylogenyanalysisindicatesthat Aphelidium is verydistantlyrelatedto Amoebaphelidium, highlightingthewidegeneticdiversityoftheaphelids.Theparasitoidencystsand penetrates thehostalga, Tribonemagayanum throughaninfectiontube.Cystgerminationleadsto a youngtrophontthatphagocytesthealgalcellcontentandprogressivelydevelopsaplasmodium, whichbecomesazoospore-producingsporangium. Aphelidium aff. melosirae has amoeboflagellate zoospores, tubular/lamellarmitochondrialcristae,ametazoantypeofcentrosome,andclosedorthomi- tosis withanintranuclearspindle.Thesefeaturestogetherwithtrophontphagocytosisdistinguish Aphelidium fromfungiandsupporttheerectionofthe new superphylumOpisthosporidiaassisterto fungi.