Ecological studies were carried out in the remaining habitat for medaka (Oryzias latipes), a marsh in the suburbs of Qinhuangdao City, Hebei Province, China. Sewage released from villages increases the nutrient levels in open water areas of the marsh, while in cattail (Typha angustifolia) and reed (Phragmites communis) beds the nutrient levels are decreased. There are fewer zooplanktons in the cattail and reed beds than in the open water areas. Sampling fishes with trap nets showed that medaka lived in dense populations in the cattail and reed beds, while the other three fish species, wild goldfish (Carassius auratus), topmouth gudgeon (Pseudorasbora parva), and mud loach (Misgurnus bipartitus), occupied the open water areas in the marsh. A little amount of paradise fish (Macropodus chinensis) was also found in the marsh. Japanese freshwater shrimps (Macrobrachium nipponense) were found only in the influx canal from waterworks. Indoor aquarium experiments showed that the wild medaka in this marsh was easily cultured with commercial feed similar to domesticated strains. Some ecological aspects of the medaka were discussed based on the observations of the fish in the marsh.