Assessments of the biological control potential of muddy loaches, Misgurnus mizolepis, were conducted against Aedes togoi, Culex pipiens pallens, and Culex inatomii larvae both in the laboratory and in small rice plots in Pusan, Korea, from June to September 1997. Medium-sized fish consumed almost all of 500 3rd-stage larvae of Cx. pipiens pallens offered daily in the laboratory. Predation by the fish at release rates of 1, 2, 4, 8, and 16 fish per plot resulted in 1,004, 1,197, 1,198, 1,200, and 1,200 larvae of Ae. togoi consumed in 12 h, respectively, when each 1,200 3rd-stage larvae were given in the 1-m2 small rice plots. The average numbers of larvae eaten after introduction of the fish were 1,121.8 larvae in 24 h and 1,195.8 larvae in 36 h. Muddy loaches showed slightly higher predation rates on Cx. pipiens pallens and Culex inatomii larvae than on larvae of Ae. togoi. Therefore, M. mizolepis might be an efficient biological control agent against mosquito species in rice fields.