Using mark-recapture methodologies, we estimated the number of adult Wood Turtles (Glyptemys insculpta) inhabiting an agricultural site in southern Québec, Canada. Two estimates, made seven years apart, revealed that the adult population declined by 50% during this period. Different hypotheses likely to explain this situation are discussed, and, although none seems more likely than the others, our data suggest that this decline resulted from increased adult mortality and poor recruitment.