The aim of this study was to determine the degradation in the rumen for the dry matter (DM), organic matter (OM) and crude protein (CP) of alfalfa (Medicago sativa L.) harvested at different maturity stages by using an in situ nylon bag method. Maturity and harvesting effects were observed in ruminally degradable DM, OM and CP. High correlations between in situ degradability and maturity were determined. Alfalfa harvested in the bud period exhibited more extensive ruminal DM, OM and CP degradation than that harvested at other maturity stages. The alfalfa hay's DM, OM and CP degradability were lowest (62.18%, 59.52% and 76.31%, respectively) in its most mature period of seed setting; its degradability decreased with maturity (17%, 14% and 13%, respectively).