An epizootic of pneumonia in 20 captive Rocky Mountain bighorn sheep (Ovis canadensis) is described. The sheep were maintained in large paddocks for abcut 9 months after which, in the late summer, the entire herd died within a three week period.

Large numbers of Muellerius adults, eggs and larvae were in the lungs of all sheep. First stage larvae were widely disseminated throughout the lungs and apparently elicited a granulomatous pneumonia. Pasteurella sp. and other bacteria were isolated from the lungs of several sheep but no Chlamydia, Mycoplasma or viral agents were recovered.

Elevated serum fibrinogen levels and normal leukocyte values were found in blood samples taken from several sheep 7-14 days prior to death.