Molecular, morphological, and cytological evidence show that the arbuscular mycorrhizal fungus Glomus scintillans is more closely related to the genera Gigaspora and Scutellospora (Gigasporaceae) than it is to Glomus s. str., and consequently it contributes to the non-monophyly of the genus Glomus. We transfer the species to Gerdemannia gen. nov., with consequent autonymy of an appropriate family, Gerdemanniaceae fam. nov. Small subunit (SSU) rDNA analysis places Gerdemannia in a clade sharing common ancestry with the Gigasporaceae, and therefore within the Diversisporales (Glomeromycota). Glomus dominikii is considered to be conspecific with G. scintillans, and these species are synonymised. Glomus chimonobambusae is also transferred to the new genus as Gerdemannia chimonobambusae comb. nov.