During autumn, a large number of adult Culex tritaeniorhynchus Giles suddenly appeared at a park in urban Tokyo. A total of 14,069 females and 2,802 males were collected at rest by sweeping from April to November 2007, of which 99.8% of the females were collected from late September to November. The appearance of a large number of Cx. tritaeniorhynchus started on 22 September, peaked at 1,062 per collection/hr on 13 October, and gradually decreased by the end of November. A total of 565 females were marked and released on 3 October, and the total number of females in the park was estimated to be 545,225 ± 157,392 by the Lincoln index. No females were collected by human landing collections, and the parous rate of females collected by sweeping was 4.4% (41/936). Dissections showed that 85.7% (120/140) of these females were in reproductive diapause, with an ovariole stage of N or I and a primary over secondary follicular ratio ≤1.5. From January to early April 2008, only 10 overwintering Cx. tritaeniorhynchus females were collected resting. These results indicated that these females were in reproductive diapause and that some overwintered in this urban setting. This is the first field evidence of prediapause seasonal migration of this species from breeding sites to overwintering sites.