Multi-element stream sediment reconnaissance in the Hess River region of the Eastern Yukon has outlined an extensive area characterized by anomalously high molybdenum values. An accessible region in the Hess Mountains, within the high molybdenum zone, was selected for detailed study of trace element levels in stream sediment, rock, soil and vegetation. In view of the frequently observed relationship between high forage molybdenum concentrations and the incidence of copper deficiency in cattle, molybdenum concentrations in plant species likely to be consumed by caribou and moose were of particular interest. High sediment molybdenum values are characteristic of catchments underlain by dark shales and less commonly dark limestone. These rocks and associated soils are rich in molybdenum. Concentrations in vegetation growing on anomalous shaly soils are characteristically low, while most plants growing on soils derived predominantly from limestone are molybdenum-rich. The Mo-Cu status of vegetation on limey soils is typically within the range associated with molybdenum induced hypocuprosis in cattle. Low molybdenum uptake by plants on soils derived from shales likely reflects the unavailability of the molybdate anion, resulting from its adsorption onto clay minerals and sesquioxides under acidic conditions prevalent in these soils. In neutral to mildly basic environments, typical of dark limestone soils, molybdenum adsorption is greatly decreased, and therefore molybdenum is relatively available to plants. In the detailed study area soil pH values are typically similar to pH levels in associated stream water. Therefore by combining stream sediment molybdenum concentrations with stream pH data, catchments likely to contain molybdenum-rich vegetation can be predicted. Unfortunately stream pH values were not obtained in the regional survey. In view of the apparent rarity of dark limestone throughout the Eastern Yukon, however, molybdenum-rich vegetation is not likely to be particularily widespread. Wildlife in this area, therefore, is probably not significantly affected by molybdenum induced copper deficiency.