Magnaporthe grisea releases a toxin(s) into the spore germination fluid, which allow a non- pathogen, Alternaria alternata, to infect host plant tissues. The role of a toxin(s) on establishment of basic compatibility between fungus and plant was investigated by ultrastructural observation and morphometric measurement. When the effects of the toxin(s) on host plants (rice and barley) and non-host plant (finger millet) were examined, the first toxin- induced changes in rice and barley plants were the loss of the mitochondrial matrix and the disruption of mitochondrial cristae. These modifications in mitochonria were observed 1 hr after toxin exposure. However, the frequency of modified mitochondria did not increase with time after toxin treatment. Other organelles were normal even in a late stage of the toxin treatment. Such phenomena were observed in three sites (epidermal, mesophyll and phloem cells) of host plants, barley and rice. However, the effects of toxin(s) were not observed in leaves of non-host plant, finger millet. Further, mitochondrial modifications were observed in both epidermal cells of compatible and incompatible cultivars 12 hr after inoculation of M. grisea spores. These results indicated that mitochondrial modification induced by the toxin(s) was an essential event for M. grisea to  establish basic compatibility with host plants at  species level.