Resistance inductions on the leaves of cucumber plant by an arbuscular mycorrhiza Glomus intraradices were investigated. In addition, the infection structures were observed at the penetration sites on the leaves of plant inoculated with Colletotrichum orbiculare using a fluorescence microscope. The severity of anthracnose disease caused by Colletotrichum orbiculare was significantly decreased on the leaves of cucumber plant colonized with G. intraradices compared with those of non-treated control plants. As a positive control, pretreatment with DL-3-aminobutyric acid (BABA) caused a remarkable reduction of the disease severity on the pathogen-inoculated leaves. There were no significant differences in the frequency of either germination or appressorium formation of the plant pathogen between mycorrhiza colonized and non-treated plants. It was also the same on the BABA pre-treated plants. However,the frequency of callose formation was significantly high on the leaves of G. intraradices colonized plants compared to those of non-treated control plants at 5 days after challenge inoculation. On the leaves of BABA treated plants callose formation was not significantly high than those of non-treated, although the disease severity was more strongly suppressed. It was suggested that the resistance induced by colonization with G.intraradices might be related to the enhancement of callose formation at the penetrate sites on the leaves invaded by the pathogen, whereas resistance by BABA did not.