Abstract: The improved fluorescence microscopic bioassay with cells from callus of Pinus thunbergii was introduced.Toxicity of water extracts of the callus inoculated with a pathogenic bacterial strain,GcM5(-)1A,carried by pine wood nematode,Bursaphelenchus xylophilus or inoculated with mixture of this strain and axenic pine wood nematodes was studied by the bioassay method.Resultsshowed that the water extracts of the callus after the inoculation with the bacterial strain alone or only the axenic nematode alone did not show toxicity.Water extracts of the callus after inoculation with mixture of aseptic pine wood nematodes and the bacterial strain or the filtrated cell free fluid of the culture of the bacterial strain on NB medium showed stronger toxicity with no statistical difference among the 3 inoculation treatments.Those above results indicated that pine wood nematode promoted the production of phytotoxin by bacterium in the host tissue.Toxicity of cell free fluid filtrated of the culture of bacterial strain on NB medium,in which pine wood nematode had been added,was not significantly changed.Resultsalso showed that pine wood nematode itself neither produced phytotoxin nor affected the toxicity of extracts from the culture of bacterium after the addition of it.