Mycelial extracts of Pythium irregulare were processed using CF‐11 chromatography and analysed by gel electrophoresis for double‐stranded RNA (dsRNA) content. DsRNA molecules of between 1 and 6 kb were found in 33 of 39 isolates tested. The dsRNA profiles of individual isolates remained unchanged after repeated subculturing of hyphal tips, after storage under water for 2 years, or when single zoospores were used to generate subcultures. However, dsRNA profiles varied between isolates, and even within individual populations; in the one case eight different dsRNA types were recovered upon testing 17 isolates from a single wheatfield. Isometric virus‐like particles (VLPs) of approximately 45 nm were found in isolates containing dsRNA, while no such particles could be found in isolates in which dsRNA was not detected. Electrophoretic profiles of dsRNA extracted from partial purifications of VLPs were identical to those of dsRNA extracted from whole mycelium, suggesting that the dsRNA is at least partially encapsidated. Attempts to transmit dsRNA between isolates by hyphal anastomosis or by coinfection in wheat plants were unsuccessful, with only parental types being recovered. Analysis of 29 isolates representing 14 other Pythium species failed to detect dsRNA, even when co‐isolated with P. irregulare known to contain dsRNA
