The members of the genus Pythium belonging to the family of Pythiaceae, order Perenosporales, and class Oomycetes are represented world-wide. More than 200 species of this genus have been described out of which 130 have been recognized recently . Most of these live as saprophytes in various types of soil and in aquatic environments, some however cause serious plant diseases. The most common disease caused by the members of this genus is the ‘damping-off of seedlings’. Some of these can also behave as parasites of mosquito larvae  and at least one is known to be a mammalian parasite .
The most widely used taxonomic descriptions of the members of the genus Pythium are those given by Middleton , Waterhouse  and Plaats-Niterink . These taxonomic references are based on the comparison of morphological characteristics and temperature–growth relationships of different members of the genus. In recent days the morphological descriptions are increasingly supplemented by molecular characteristics of a given species . The polymerase chain reaction coupled to restricted fragment length polymorphism analysis (PCR–RFLP) has become a useful tool in fungal taxonomy [6,7] and is currently used to identify different species of Pythium[8–11]. Amplification of the ribosomal gene is used for the genetic identification of many organisms because they comprise both highly conserved sequences during evolution and highly variable sequences among species and even within species. The ribosomal nuclear DNA consists of transcribed and non-transcribed regions . The internal transcribed spacer (ITS1) is a non-conserved region and has been amplified with the PCR method using universal primers ITS1 and ITS2. The comparison of mitochondrial DNA (mt DNA) has also been used for taxonomic purposes .
During the course of investigation on pythiaceous fungi, the author described an isolate (F-66) as Pythium ostracodes Drechsler . The study on the fungus was then purely morphological and despite of some differences like the absence of zoospores, it was incorporated within P. ostracodes. Recently, P. ostracodes (F-333) having zoospores, was isolated from a soil sample taken in the Burgundy region of France. The ITS1 regions of the rDNA of both the fungi were amplified and the sequences revealed that the isolate F-66 was wrongly described as P. ostracodes. Further studies have made it possible to re-describe the isolate F-66 as a new species. The name Pythium megacarpum has been adopted due to the presence of large oogonia and thick-walled oospores. In the present paper, the morphological details of the new species together with the sequences of the ITS1 region of the ribosomal nuclear DNA are given. Comparison of morphological as well as molecular characteristics with related species are also dealt here.