We investigated the occurrence of phytoseiid mites, as indigenous natural enemies of Thrips tabaci Lindeman, in winter-harvested Welsh onion fields in Chiba Prefecture, Japan. Periodic surveys were carried out in five fields, and six species of phytoseiid mites were captured. We assumed the dominant species was Gynaeseius liturivorus（Ehara）, because we captured this species in 8 fields during temporary surveys of 17 fields. Gynaeseius liturivorus populations were higher during October to December in winter-harvested Welsh onion fields receiving lower applications of agrochemicals than conventionally controlled fields. Adult female densities were significantly higher on the insides of leaf folds than on other surface areas of the Welsh onions; all stages of G. liturivorus were frequently observed at peak abundance on the insides of the leaf folds. These results suggest that the insides of Welsh onion leaf folds provide a suitable habitat for G. liturivorus. We identified their predatory potential against T. tabaci in the laboratory（adult females consumed 1.1±0.4 adults or 5.3±1.9 2nd instar larvae per day at 22°C with a 12L:12D photoperiod）, and observed suppression of thrips in the fields at peak abundance of G. liturivorus. However, development of enhanced techniques for insectary plants and evaluation of agrochemical effects on G. liturivorus are needed in order to sustain higher mite densities, because the control of thrips by mites was not sustainable in this study.