In October, 1963, during a time of abundance of microtine rodents,Pasteurella tularensiswas isolated from a northern vole,Microtus oeconomusPallas, at the Ugashik Lakes on the upper Alaska Peninsula. The morphological, cultural, and serological characteristics of this isolate are described, and comparative virulence in experimentally inoculated animals, including series of indigenous rodents, is discussed. The isolate was less virulent for rabbits and guinea pigs than was that which has been isolated previously from ticks,Haemaphysalis leporispalustris(Packard), in Alaska, and was also less virulent for these animals than was strain SCHU S4. The isolate from the vole seemed to resemble most closely the Eurasian strain ofP.tularensis, as might be expected on zoogeographical grounds. A distinguishing feature of the isolate was its ability to grow readily on blood agar in the absence of cystine. The relatively high rate of subclinical tularemia in man in northern and western Alaska, as indicated by the results of serological tests, may be attributable to this organism. Water-borne bacteria may be the source of infection in man.