We report green fluorescent protein (GFP) as a useful reporter molecule to track the occurrence of Pseudomonas plecoglossicida in ayu Plecoglossue altivelis. The gfp gene was placed in a shuttle vector pME4510 and then three kinds of GFP vectors, pSKL01, pSKT03 and pSKN04 were constructed, in which lac, tac, and neophosphotransferase II (npt2) promoters drove the expression of GFP, respectively. A promoterless GFP vector was designated as pSKP02. P. plecoglossicida carrying pSKT03 gave the highest expression of GFP. In addition, pSKT03 was relatively stable under non-selective pressure and there was no significant changes in the in vitro growth and pathogenicity of the cells carrying this plasmid. The cells of fluorescent P. plecoglossicida attaching to the body surface of ayu were easily detected under a fluorescence microscope. It was revealed that they adhered predominantly to the microscopic injuries in the skin and fins.