Thirteen isolates of Eastern encephalitis (EE) virus recovered from various hosts in Massachusetts, USA, differed significantly in pathogenicity for pheasants, 21 days old or older, inoculated s.c. with graded doses of virus. Among the least pathogenic were 2 isolates from Culiseta melanura mosquitoes, and among the most pathogenic isolates was 1 of pheasant origin. Nine day old pheasant chicks were highly susceptible to EE virus. Mortality did not appear to be totally dose-related, for inocula larger than an infecting dose did not increase mortality substantially.