Two new arbuscular mycorrhizal fungal species, Glomus achrum sp. nov. and Glomus bistratum sp. nov. (Glomeromycota), are described and illustrated. Both species produce small, hyaline spores in aggregates formed in the soil and inside roots. Glomus achrum was associated with roots of Ammophila arenaria ( L.) Link colonizing maritime dunes of the Vistula Bar in northern Poland, and G. bistratum occurred among vesicular-arbuscular mycorrhiza of Xanthium cf. spinosum growing in dunes of the Mediterranean Sea adjacent to Veriko, Greece. Spores of G. achrum are globose to subglobose, (25-)43(-55) mu m in diameter, rarely egg-shaped, oblong to irregular, 15-45 mu m x 55-65 mu m. Their wall consists of three hyaline layers: a mucilaginous, short-lived outermost layer; a laminate middle layer composed of loose sublayers; and a flexible innermost layer. The outermost and the innermost layers stain deeply red in Melzer's reagent. Spores of G. bistratum are globose to subglobose, (20-)29(-50) mu m in diameter, and have a wall composed of two permanent, hyaline layers. The outer layer is unit, smooth, and the inner one laminate. Only the inner layer stains yellow in Melzer's reagent. Both species formed vesicular-arbuscular mycorrhiza in single-species cultures with Plantago lanceolata L. as the host plant. Phylogenetic analyses of partial 18S rDNA subunit and internal transcribed spacer (ITS) region sequences placed G. achrum and G. bistratum into Glomus group A, but did not reveal any closely related described species. Environmental sequences from the public databases suggested that G. achrum occurred in at least two other plant species from geographically distant regions. No such evidence could be obtained for G. bistratum, which is currently known only from the type location.