A one-year preliminary sampling of pesticide practices on the Flathead Indian Reservation was conducted, with later work concentrating on the effects of DDT on osprey survival and nesting on Flathead Lake, Montana. The first year's sampling program indicated that major surface, underground and culinary waters were contaminated with Tordon and probably other pesticides and/or herbicides. The phenomenon was believed responsible for the catastrophic decline of the populations of many predatory birds, with evidence that it might also be responsible for the decline of the Flathead Lake osprey population. The later work provided additional confirmation of the osprey population decline and delineated possible reasons. Reduction in food resources did not, however, appear to be significant, nor was there evidence of the decline in population being a result of human interference.