construct containing reporter gene(pFV4CAT) regulated by carp $eta$-actin promoter was microinjected into the one-cell stage egg of mud loach (Misgurnus mizolepis), and was successfully expressed, possibly by the integration into the genome. Both mean hatching success and early survival of the microinjected groups were not significantly different with those of control groups (P>0.05). The incidence of transgene was ranged from 7 to 48% based on the PCR and/or Southern blot analyses with the DNA prepared from fin or blood tissue. The spatial and temporal patterns of expression of the pFV4CAT gene, measured by in situ immunohistochemical analysis peroxidase-conjugated anti-CAT antibody, were variable among the experimental individuals. These results suggest that carp $eta$-actin promoter is effective to express other transgene in mud loach, such that this promoter can be useful in the generation of valuable transgenic mud loach.