A strain of P. Aeruginosa,which was seperated from clinical environment ,shows a special characteristic . It keeps normal short rod shape when cultured at 37℃,however,it forms filament without pyocyanin producing when cultured at 25℃ overnight. The filaments will divide and form short rods,simultaneously,produce pyocyanin when culture time is prolonged to over 72h or culture temperature is raised to 37℃. The preliminary study indicates that this phenomenin has nothing to do with nutritive conditions and could the inbluenced by inoculating density and irradiating with ultraviolet rays The absence of pyocyanin was not the cause of filamentous formation by the test results.