A monoclonal antibody (MoAb) API9-2, which reacts specifically to members of the genus Fusarium, was tested for serological detection of F. oxysporum in plant tissue. Mycelia of F. oxysporum in infected stems, crowns, and roots of tomato and Japanese honewort plants were effectively detected by immunofluorescence assay (IFA) with the combination of MoAb API9-2 and FITC-conjugated anti-mouse IgM-sheep IgG indicating that IFA is a useful tool for the observation of behavior of Fusarium spp. in plant tissue. As another immunological assay with this MoAb, direct tissue-blotted immunobinding assay (DT-IBA) was applied to transverse sections from stems or crowns of tomato and cucumber plants. In this assay, samples of plants infected with F. oxysporum showed positive reaction within 4hr. In addition, the pathogen residing in vessels of infected, but symptomless plants was observed within 14 to 21 days after inoculation. Thus, DT-IBA is thought to be a practical method for quick and accurate diagnosis of Fusarium spp.