Six strains of virus were recovered from the blood and/or liver of five Clethrionomys gapperi ochraceus trapped in southeastern Massachusetts during 1969. Biological, antigenic and physiochemical properties of these isolates are reported. USA M-2268a was selected as the reference strain. This strain was identical by complement-fixation and neutralization tests to Whitney's C. gapperie virus (USA 64-7855) from New York State and was related to, but distinct from, an unpublished agent (Johnson's Microtus montanus enterovirus USA M-1146) isolated in June, 1962 from voles trapped in Klamath County, Oregon. USA M-2268a was resistant to lipid solvents and acid pH and was stable at temperatures of 4 C, 22 C, and 37 C. Virus was detected over a 10-day observation period in four species of mosquitoes inoculated with USA M-2268a, although there was no evidence of infection or replication, and transmission attemps by bite failed. Neutralizing antibody was detected in C.g. gapperi and C. g. ochraceus in various habitats throughout the state.