Epichloë typhina is a clavicipitaceous ascomycete which systemically infects grasses, causes choke disease of host inflorescences, and is related to a group of mutualistic grass endophytes. Three plasmids of 7.5, 2.1 and 2.0 kilobase pairs were found in mitochondrial DNA preparations of an E. typhina isolate from perennial ryegrass (Lolium perenne). Results of nuclease digestion indicated that the plasmids, designated Et7.5L, Et2.1L, and Et2.0L, were linear, double-stranded DNAs with protein linked to their 5'-ends (plDNA). The plasmids shared little or no homology with each other, and were not integrated into the mitochondrial or nuclear genomes. No homologous plasmids were detected in isolates of E. typhina from other grass hosts, anamorphic endophytes, or other Clavicipitaceae. However, other plasmids were present in Balansia obtecta and Claviceps purpurea. A partial sequence of one of the E. typhina plasmids, Et2.0L, indicated an open reading frame when UGA was assumed to encode tryptophan. The inferred amino acid sequence had 24% identity over 258 amino acids in two regions of the reverse transcriptase encoded by the circular Mauriceville and Varkud plasmids of Neurospora spp. The homologies included six segments conserved in RNA template-dependent DNA or RNA polymerases.