Pythium rhizo-oryzae sp. nov. was isolated from some soil samples taken in paddy fields in northeastern India. For quite a long time it was wrongly identified as Pythium catenulatum Mathews, because of the presence of numerous catenulate hyphal bodies. The sequencing of the ITS region of its nuclear ribosomal DNA and a close look at its morphological characters have now enabled us to describe it as a new species. This species is characterized by the absence of zoospores and sporangia, rarely formed aritheridia and oogonia, and the presence of chained and terminal hyphal bodies. The ITS region of its rDNA comprises 774 bases. This oomycete is closely related to the members producing a filamentous inflated type of sporangia like P. catenulatum Mathews, P. torulosum Coker & Paterson, and P. inflatum Mathews. The closest of all these is P. catenulatum. Taxonomic description of this new species and its comparison with related oomycetes, together with the sequence of the PCR amplified internal transcribed region (spacers ITS1, ITS2, and the gene 5.8S) of its rDNA are given here.