The life history of the big-eyed bug, Piocoris varius, was investigated in two locations in Ibaraki and Chiba, Japan in 2004 by observing seasonal changes in the stage composition of field populations. The nymphal instar of field-collected samples was identified by comparing their head capsule widths with those of laboratory-reared individuals. P. varius nymphs and adults were found on 19 plant species (12 families) of 24 species (14 families) on which the population census was made. Overwintering adults became active on plants in May and disappeared in July. First-generation young nymphs appeared in late May and June. They developed into old nymphs in late June and then emerged as adults in late July and August. After August, adults predominated until December, although a small proportion of second-generation nymphs occurred in August and September. Only adults were found under the litter in December. These observations indicate that P. varius is basically a univoltine species with a partial bivoltine lifecycle in the Kanto district in Japan.