A new fungal species of the Glomeromycetes, Acaulospora aspera, was isolated from the rhizosphere of the inka nut (Plukenetia volubilis) in San Martin State of Peru (Western Amazonia) and propagated in bait cultures on Sorghum spp., Brachiaria brizantha, Medicago sativa and P. volubilis as host plants. The fungus forms brownish yellow to yellow brown spores, (120-)135-195 x (120-)130-187 mu m in diameter. The surface of the structural spore wall layer is crowded with small depressions, 0.4-0.7 mu m in diameter, up to 0.8 mu m deep, and only 1.1-1.8 apart, giving the spore surface a rough, washboardlike appearance, especially when the outermost, evanescent wall layer has disappeared. Phylogenetically, the new species is close to A. spinosissima, A. excavata and to other morphologically more similar species such as A. spinosa and A. tuberculata, which form spiny or tuberculate projections ...