Physical and biological attributes of water channels utilized by immatures of Culex pipiens pallens were studied in Saga City, Japan. Water in mosquito-productive segments generally was stagnant or slowly running (mean < 5 cm/sec), with low and fluctuating dissolved oxygen concentrations and high electric conductivity. Water flow > 20 cm/sec was considered necessary for prevention of mosquito breeding. The aquatic fauna in mosquito-productive segments was composed of taxa tolerable to polluted water. Adult Odonata were more diverse in segments with emergent vegetation irrespective of physical attributes of channel water. Fish diversity was higher in mosquito-free segments. Twenty-three fish species were confirmed in the creek networks. Temporary flooding did not flush mosquito immatures from mosquito-productive segments, indicating the high stability of those segments as mosquito immature habitats.