Transitions from unicellularity to multicellularity have occurred several times in the history of life. Two of the most conspicuous multicellular kingdoms, Metazoa and Fungi, form a monophyletic clade, known as the opisthokonts, which comprises some unicellular lineages, such as Choanoflagellata, Nucleariidae, Ichthyosporea (also known as the DRIPs or Mesomycetozoea), Ministeria and Corallochytrium[1.Cavalier-Smith T. Chao E.E. Phylogeny of Choanozoa, Apusozoa, and other protozoa and early eukaryote megaevolution.J. Mol. Evol. 2003; 56: 540-563Crossref PubMed Scopus (193) Google Scholar, 2.Steenkamp E.T. Baldauf S.L. Origin and evolution of animal, fungi and their unicellular allies (Opisthokonta).in: Hirt P. Horner D.S. Organelles Genomes and Eukaryote Phylogeny. The Systematics Association Special Volume, series 68, CRC Press, London2004: 109-129Google Scholar]. Among these unicellular opisthokonts we should expect to find the closest extant relatives of the multicellular Metazoa and Fungi. Choanoflagellates have traditionally been considered to be closely related to metazoans [1.Cavalier-Smith T. Chao E.E. Phylogeny of Choanozoa, Apusozoa, and other protozoa and early eukaryote megaevolution.J. Mol. Evol. 2003; 56: 540-563Crossref PubMed Scopus (193) Google Scholar, 2.Steenkamp E.T. Baldauf S.L. Origin and evolution of animal, fungi and their unicellular allies (Opisthokonta).in: Hirt P. Horner D.S. Organelles Genomes and Eukaryote Phylogeny. The Systematics Association Special Volume, series 68, CRC Press, London2004: 109-129Google Scholar, 3.King N. Carroll S.B. A receptor tyrosine kinase from choanoflagellates: molecular insights into early animal evolution.Proc. Natl. Acad. Sci. USA. 2001; 98: 15032-15037Crossref PubMed Scopus (168) Google Scholar, 4.Snell E.A. Furlong R.F. Holland P.W. 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USA. 2004; 101: 8066-8071Crossref PubMed Scopus (193) Google Scholar]. Nuclearia sp. (ATCC 30864), recently renamed as Capsaspora owczarzaki, a filose amoeboid symbiont of a pulmonate snail, has never been confidently assigned to any opisthokont lineage [7.Amaral Zettler L.A. Nerad T.A. O'Kelly C.J. Sogin M.L. The nucleariid amoebae: more protists at the animal-fungal boundary.J. Eukaryot. Microbiol. 2001; 48: 293-297Crossref PubMed Scopus (67) Google Scholar, 8.Hertel L.A. Bayne C.J. Loker E.S. The symbiont Capsaspora owczarzaki, nov. gen. nov. sp., isolated from three strains of the pulmonate snail Biomphalaria glabrata is related to members of the Mesomycetozoea.Int. J. Parasitol. 2002; 32: 1183-1191Crossref PubMed Scopus (59) Google Scholar, 9.Medina M. Collins A.G. Taylor J.W. Valentine J.W. Lipps J.H. Amaral-Zettler L. Sogin M.L. Phylogeny of Opisthokonta and the evolution of multicellularity and complexity in Fungi and Metazoa.Int. J. Astrobiol. 2003; 2: 203-211Crossref Scopus (62) Google Scholar]. Here, we report large subunit ribosomal and actin sequences of Capsaspora owczarzaki and an actin sequence of the novel ichthyosporean species Sphaeroforma arctica[11.Jostensen J.P. Sperstad S. Johansen S. Landfald B. Molecular-phylogenetic, structural and biochemical features of a cold-adapted, marine ichthyosporean near the animal-fungal divergence, described from in vitro cultures.Eur. J. Protistol. 2002; 38: 93-104Crossref Scopus (25) Google Scholar]. We test the phylogenetic position of Capsaspora owczarzaki, and the phylogenetic position of the nucleariids within the opisthokonts.