>From Johnson, R. E. and K. M. Cassidy. 1997. Terrestrial mammals of
Washington State: Location data and predicted distribution. Volume 3 in
Washington State Gap Analysis - Final Report (K. M. Cassidy, C. E. Grue, M.
R. Smith, and K. M. Dvornich, eds.) Washington Cooperative Fish and Wildlife
Research Unit, University of Washington, Seattle, 304 pp.

"The Virginia Opossum had entered Washington by 1941 by deliberate
introduction from the eastern U.S., migration from introduced populations in
Oregon, or both. However, much of its population increase and range
expansion in Washington appears to have occurred since the mid-1980s. The
Opossum reached high numbers in the Puget Trough first and is now common
throughout the Puget Trough from the Columbia River to the Canadian border.
It was not known from Thurston County until 1982 or 1983 but is now common
there. It also occurs along the coast from the mouth of the Columbia River
north to Cape Flattery and probably along the coast from Cape Flattery east
to the Puget Trough. It was first noted at the mouth of the Columbia River
about 1985. On the west side, it generally occurs below 1000 feet in
broadleaf forestsalong streams and rivers and on farmland. The Opossum is
known from Bainbridge Island, but, so far as is known, has not reached the
San Juan Islands or Guemes, Cypress, Lummi, Whidbey, Vashon, or Camano
Island.
"The Opossum's range expansion has been slower in eastern Washington. The
first documented occurrence on the east side (a 1958 museum record) was in
the Malaga-Wenatchee area of Chelan County. Since the early 1990s, Opossums
have been reported along the Yakim River from Union Gap south, and they
probably occur in the Yakima area as well. They wer well-established in
Umatilla County in Oregon (opposite Walla Walla County in Washington) by
1910, yet the first reports of Opossums along the Walla Walla River and Mill
Creek (by trappers) are from ~1993. The first report of an Opossum from
Spokane was also in 1993, when a wildlife rehabilitator captured one in the
city and later released it there."

The information about the earliest Opossum records are from:
Scheffer, V. B. 1943. The Opossum settles in Washington State. Murrelet
24:27-28.

The information about recent spread and distribution was based on
conversations with Dinah Demer, Ellen Kritzman, Roger McKeel, Pat Mowler,
Joe Piecuch, Greg Schirato, and Jeff Skriletz. Most of those people (except
Ellen Kritzman and Joe Piecuch) are with the WA Dept. of Fish and Wildlife.

An addendum to the above: Steve Erickson (Frosty Hollow Ecological
Restoration) sent me an email in August 1988, saying that Opossums now occur
throughout Whidbey Island.

Kelly Cassidy
Washington Cooperative Fish and Wildlife Research Unit
University of Washington
lostriver at seanet.com