Achlya anomala sp. nov. is described from acetate sheets put in Las Cańas stream as a substratum, in Buenos Aires Province (Argentina). The species is illustrated and compared with other species of the genus; it develops characteristic large swellings like anomalous oogonia intermingled with the normal ones, often one or several times proliferating, with or without oospores inside. The oogonia are smooth or with scanty papillate or tuberculate projections; the antheridial branches are usually diclinous, often monoclinous and rarely androgynous; oospores are eccentric, (1)2–12(20) per oogonium.
Keywords: Achlya, taxonomy, straminipilous fungi, freshwater.