Morphological analysis of a small freshwater goby, Rhinogobius flumineus, showed a distinct dimorphic asymmetry in the lower jaw. The mouth of each fish opened with a slight but definite distortion toward the right or left side, depending on the individual. Right-opening mouth (dextral) fish had a right lower jaw that was more protruded than the left one, and left-opening (sinistral) fish had a more protruded left lower jaw. No fish had laterally symmetric lower jaws, indicating that the asymmetry was different from ‘fluctuating asymmetry’. These fish used either the right or left side of the mouth when picking up food from the bottom, but neither dextral nor sinistral individuals used one side more frequently than the other side. The mouth asymmetry, however, was related to the stationary posture: dextral fish stayed on the bottom with the line of their bodies curved to the right more frequently than to the left, and vice versa for the sinistral fish. Genetics of the morph phenotype were investigated by observing the frequencies of morphs in F₁ under captive breeding. Mouth dimorphism seems to be determined by the Mendelian one-locus-two-alleles system, in which dextrality is dominant over sinistrality and the dominant gene acts as the lethal one when in a homozygote.