The relation between macroconditions of water channels and the breeding of Culex pipiens pallens in Saga City were examined, based on weekly census data over 3 years for ca. 2,000 channel segments. Mosquito breeding activity was greatest in channel segments between building lots and field lots in the suburban area. The segments involved ranged from narrow roadside ditches (width < 1 m) to wide irrigation canals (width > 10 m), all of which were utilized by mosquitoes. There was a weak tendency for more mosquito breeding in narrower segments. Water flow indices ranged from 0% (always stagnant) to 100% (always flowing). Mosquito breeding tended to be suppressed with an increase in flow indices. Mosquito breeding was confirmed in ca. 35% of segments yearly, and such mosquito-productive segments tended to persist during 3 study years.