Pythium carbonicum (F-72) sp. nov. was found in soil samples taken on the top of a spoil heap in northern France. The morphology of this new species resembles that of a recently described species: Pythium megacarpum. However, the antheridial and oogonial characteristics of this new species are unique, and the comparison of its ITS region of the nuclear ribosomal DNA indicates that this species is also related to the genus Phytophthora. The fungus does not sporulate, the sporangia germinate directly into mycelium through germ tubes. The oogonia of P. carbonicum are smooth-walled and also papillated, and are provided with monoclinous and diclinous antheridia that wrap around, forming a complicated knot. Morphological features of this new species, together with the sequences of the ITS region of its nuclear ribosomal DNA and its comparison with related species are discussed here.