Based on concomitant molecular analyses of the ribosomal gene and morphological characteristics, we divide the phylum Glomeromycota into three classes: Glomeromycetes, Archaeosporomycetes, and Paraglomeromycetes. Glomeromycetes are newly organized in three orders: Glomerales and Diversisporales, both forming typical vesicular arbuscular mycorrhiza with higher plants, and Gigasporales, forming arbuscular mycorrhiza without vesicles in the roots but with extra-radical auxiliary cells. Within the phylum, Archaeosporomycetes comprise exclusively bimorphic families and genera. The monogeneric Paraglomeromycetes species form glomoid spores that typically germinate directly through the spore wall instead through their subtending hyphae.