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The research was conducted at Bukit Soeharto, East Kalimantan, Indonesia. In the natural forests there were as many sound as diseased trees. There were no clear differences in disease incidence among forests with different intensities of cutting and among different diameter classes. Observations were made on 49 trees of 15 species in sample plots. Among them only Dipterocarpus convertus, D. cornutus, Shorea leprosula, S. ovalis and S. seminis remained sound. Stem canker was judged to be the most dangerous disease. Invasion of disease in uncut forest (control) was low level compared with cutting treatments, but difference of disease invasion between the forests with different cutting intensities was not clear because of low numbers of sample trees. Sound trees should be preferred to diseased trees for seed trees. In the taungya plantation there were generally more sound plants of all species compared to diseased and dead ones, except for Manihot esculenta and Glycine max which were more diseased than sound. Leaf spots were found in all species of plants. Among the forestry plants, the highest incidence of leaf spot disease occurred in Hevea brasiliensis and among the agricultural plants, the highest incidence was in G. max. Generally, invasion of diseases in M. esculenta and G. max caused moderate damage, while other species had a low level of damage. Mixed planting in the taungya system presents no problem from disease viewpoint because of the specificity of the diseases.