The Anobiid beetles Stegobium paniceum and Lasioderma serricorne possess the intracellular yeastlike symbionts Symbiotaphrina buchneri and Symbiotaphrina kochii, respectively, in the mycetome between the foregut and midgut. The nucleotide sequences of the small-subunit rRNA-encoding genes of the symbionts were determined for phylogenetic analysis. Five group I introns were found in the small-subunit rRNA-encoding genes of S. buchneri, but S. kochii lacked introns. The phylogenetic position of both symbionts was close to the class Discomycetes, which is a paraphyletic group. The two symbionts formed a monophyletic group distinct from the other major lineages. Both appear to have parted from other filamentous fungi during the early radiation of the euascomycetes and to have later become obligatory partners of the beetles.