Particles of a poxvirus found in Californian western grey squirrels have overall dimensions indistinguishable from those of the poxviruses of the genera Orthopoxvirus and Leporipoxvirus, but have surface structures somewhat reminiscent of virions of members of the genus Parapoxvirus. Extracts of tissues infected with the squirrel poxvirus cross-react in gel-diffusion tests with extracts of tissues infected with Californian myxoma virus, but not with a similar preparation infected with a South American strain of myxoma virus.