Neoseiulus californicus (McGregor), which is a potential biological control agent of spider mites, has been produced by some Korean natural enemy companies. Now, our project related to the biological control of spider mites by releasing N. californicus in apple orchards is going on. The field occurrence of this species in Korea was first reported in Jeju citrus orchards (Jung et al. 2006). However, the natural occurrence of the mite has not been recorded in Korean mainland so far. Therefore, it is questioned whether the released N. californicus population originated in Jeju can successfully overwinter in Korean mainland. First, to clarify the natural overwintering sites in field, we conducted field samplings in Jeju citrus orchards in Feb. 2008. Samples were collected from the various possible overwintering habitats, such as citrus leaves and twigs, ground covers and windbreaks. Most of the mites were collected from ground cover samples. Next, to estimate of the winter mortality, the mite was individually put in small tubes which have some different food conditions, and the tubes were kept under the different ground surface conditions in the university experimental orchard in Dec. 2007. Most of the mites had died by late Jan. and all by early Mar. 2008. And also, to evaluate the field survival of the mite, in the spring 2008, sampling would be conducted in apple orchards in Cheongsong where we released the mites in 2007. From these data, we would discuss possible overwintering of N. californicus in Korea.