Halobates spp. are the only insects inhabiting the open sea. One sea skater species, Halobates sericeus, was collected at 18 locations in the East China Sea area (27°10´N- 33°24´N, 124°57´E-129°30´E), and H. micans and/or H. germanus at only 8 locations in the area south of 29°47´N, where water temperatures were more than 25°C. At three locations, where the water temperature was less than 23°C, neither H. micans nor H. germanus were caught. The effect of photoperiod on the aggregation and mating behaviour of the sea skater, H. sericeus, was studied under laboratory conditions during a one-month cruise. Adults and 5th instar larvae of H. sericeus, collected between 29°02´N and 30°29´N, were kept under long (14.5L : 9.5D) or short-day (10.5L : 13.5D) conditions at 23 ± 2°C for 20 days. Aggregation and mating behaviour of these sea skaters were recorded over a period of 150 min during the daytime. To analyze the data, the observation period of 150 min was divided into 50 intervals of 3 min. Aggregation was observed more under short than long-days. Duration of a group was much longer under the short (mean ± SD : 43.0 ± 108.1 sec) than long-days (7.6 ± 3.1 sec).