Analysis of 18S rDNA sequence and morphological data indicate that Achlya s. lat. is polyphyletic. Aplanopsis spinosa and Achlya species with subcentric oospores and laterally applied antheridia are transferred to Newbya gen. nov. The relationship of Calyptralegnia and Aplanes androgynus to Newbya is discussed. Molecular data support morphologically based conclusions concerning the relatedness of Pythiopsis and Aplanopsis terrestris. Achlya truncatiformis nom. nov., is provided for material previously identified as Achlya recurva. The following new combinations within Newbya are made: N. pascuicola (type species), N. apiculata, N. braziliensis, N. curvicollis, N. megasperma, N. oblongata, N. oblongata var. gigantica, N. oligacantha, N. polyandra, N. recurva, N. spinosa, and N. stellata combs nov.