SUMMARY1.The conidia and germ tubes ofHelminthosporium sativumare multinucleate, whereas hyphal cells are uninucleate to multinucleate, and young conidiophores are usually uninucleate. All nuclei in the conidia produced on the same conidiophore are perhaps derivatives of a single nucleus and hence genetically alike.2.True hyphal fusion between races ofH. sativumis common but rare betweenH. sativumand other dark-spored species. However, pseudo-hyphal fusion between species ofHelminthosporiumis not uncommon.3.Cultural comparisons were made of 207 hyphal-tip isolates obtained from 103 germinating conidia taken from colonies giving rise to variants or from mixed colonies on agar drops. All the isolates from a single conidium were identical with the original culture.4.Cultural comparisons were made of 524 conidial isolates obtained from 205 individual conidiophores taken from mixtures of 2 distinct races or species growing on agar drops or from colonies derived from barley tissues infected with 2 or more distinct races or species ofHelminthosporium.All the isolates from the same conidiophore, with two exceptions, were identical culturally.5.Variants occur frequently inHelminthosporiumspp. either in the form of sectors or“patches.”AlthoughH. sativumrace 1 has been cultured for 17 years, it still gives rise, under certain conditions, to numerous variants of different types.6.Potato-dextrose agar rendered stale by certain bacteria was found conducive to the development of variants.7.The results indicate that genetic variation inH. sativumis caused primarily by nuclear change rather than by heterocaryosis.