An interdisciplinary group of faculty from the University of Colorado and from Colorado State University is studying molybdenum in the environment. Molybdenum plays an essential role in the nitrogen cycle of plants and may cause disturbance of copper metabolism in animals. The world's largest molybdenum-producing mine is at Climax, Colorado. Rivers in Colorado exhibit some of the highest reported concentrations of molybdenum in the United States. Colorado offers a model system for the study of the release and effect of molybdenum. The geochemistry of molybdenum is complex. The principal dissolved form of the metal in natural waters is an anion, MoO 4 − − . At values of pH below about 6, the bimolybdate ion, HMoO 4 − , becomes dominant. The bimolybdate ion is relatively immobile in natural systems at low pH, probably because of adsorption or coprecipitation on metal hydroxides. In the acid soils of the alpine environment of Colorado, molybdenum forms a well-defined halo of elevated concentrations around a mineralized, undisturbed zone in the bedrock. We have attempted to define a natural datum or background level of molybdenum in the vicinity of the undisturbed mineralized zone and to compare the concentrations of molybdenum in the undisturbed area to those present in the vicinity of mines and mills in the same mountainous area. Such a comparison is extremely difficult and tenuous because of differences in drainage and glaciation between the two areas.