Seasonal changes in population density and species composition of Orius spp., predacious natural enemies of thrips, and population density of Pseudodendrothrips mori were investigated in mulberry fields in 1999 and 2000. Orius spp. adults and nymphs occurred in mid May and late May and in late May and early June in 1999 and 2000, respectively. These first appearances were later than those of P. mori adults and larvae. The peak periods in population density of Orius spp. accorded with that of P. mori from June to September. Adults and nymphs of Orius spp. showed random distribution as individuals and contagious distribution as individuals per branch, respectively. Adults and larvae of P. mori showed contagious distribution as individuals and contagious distribution as colonies per leaf, respectively. O. sauteri, O. minutus and O. strigicollis were always found cohabitated in mulberry fields. The numbers of 2nd instar P. mori larvae consumed by three Orius species female adults and 5th and 3rd instar nymphs were about 33, 26 and 21 per day, respectively. From these results, it is suggested that these three Orius species are important natural enemies of P. mori in mulberry fields.