Tolerance to salt water of a salt marsh population of the meadow vole, Microtus pennsylvanicus, was compared with that of an inland population in Connecticut. Tolerances were determined by survival when maintained on water increased in salinity in steps of 0.05 m NaCl at 1-, 5-, and 10-day intervals. One group was also placed directly on 0.35 m NaCl. There was no significant difference between the two groups in their ability to utilize salt water. Neither population could tolerate salinities above 0.30 m, approximately one-half sea water, for even short periods. Observations indicated there would be times when no free fresh water (in the form of dew, rain, or snow) would be available in the marsh. Measurements of water content and salinity of the common marsh plants indicated the voles could obtain usable water from Spartina patens. The other sedge common in the marsh, Spartina alterniflora, had too high a salinity (approximately 0.35 m NaCl) to be utilized.