Active oxygen accumulation in wheat and powdery mildew fungi infection was examined with DAB reaction. The results showed that active oxygen was detectable on all the infected sites by the fungal primary germ tubes, appressoria, and the infection lobes of growing secondary hyphae. In the compatible interaction, wheat epidermal cells produced relatively less active oxygen; The active oxygen was relatively high on the cells infected by appresorium firstly, and there was no enough activeoxygen accumulated to be detected white a haustorium-containing cell was infected again by infection lobes of the hyphae. In the imcompatible interaction and the nonhost interaction between wheat and barley powdery mildew pathogen, much more active oxygen was examined, especially in the nonhost resistant interaction. It was found that, however, more active oxygen was accumulated in short epidermal cells than that in the long ones, even the long cells were more resistant than the short ones were. These suggested that active oxygen just is one of the symbols of plant defense response in wheat against powdery mildew, but it is not the decisive and the exclusive factor of resistance.