Selective isolation procedures were carried out on decaying leaves of Nymphoides peltata (Gmel.) O. Kuntze in order to study the pythiaceous fungi (Peronosporales) associated with the decomposition processes. In addition to Pythium marsipium Drechsler and P. pleroticum T. Ito, a number of Pythium species were isolated with filamentous non-swollen sporangia. Under special culture conditions (in water under a day/night rhythm) sexual reproductive structures of P. diclinum Tokunaga and P. apleroticum Tokunaga were also obtained. However, several isolates designated as Pythium ‘F’, remained sterile. Within this group three types could be distinguished according to their temperature-growth relationships. As the species recorded have been found very rarely and only one was represented in the CBS collection, detailed descriptions with short discussions are presented.