Several Neotyphodium species (asexual fun- gal symbionts of grasses) have been characterized ge- netically as heteroploids (nonhaploids) derived by in- terspecific hybridization involving sexual Epichloiespe- cies (Clavicipitaceae, Hypocreales). Examples in- clude Neotyphodium coenophialum with at least three ancestral Epichloe species, and a rare endophyte of perennial ryegrass that appears to be a hybrid of Ep- ichloe typhina with the common perennial ryegrass endophyte, Neotyphodium lolii. In contrast, N. lolii ap- pears to be haploid and closely related to Epichloe festucae. Hypothesizing that hybrid origins should re- sult in larger genome sizes, we estimated genome siz- es for two Epichloe and three Neotyphodium spp. Using quantitative Southern blot hybridization we estimat- ed genome sizes of 29 ? 4 Mb for two sexual hap- loids, E. festucae isolate E189 and E. typhina E8. Es- timates for hybrid anamorphs were 57 ? 7 Mb for N. coenophialum el9, and 55 ? 7 Mb for N. lolii X E. typhina Lpl. Electrophoretic karyotyping of these four isolates corroborated the genome size estimates. Also, the electrophoretic karyotype of N. lolii isolate e45 indicated a genome size similar to that of the Epichloe species, in keeping with the apparent non- hybrid nature of N. lolii. Genome size differences were reflected in the variation of conidium lengths and widths suggesting that large conidial size may be an indicator of heteroploidy and, therefore, possible