Abstract In a study of stigma receptivity in Acer saccharum Marsh., conducted under greenhouse conditions, germination of pollen grains on stigmas was first observed about 22.5 hours after bud burst. The height of stigma receptivity, as measured by pollen-grain germination and pollen-tube growth, occurred at 38 hours after bud burst. Pollen-tube growth also was first observed 22.5 hours after bud burst, but growth did not occur after about 95.5 hours. The pollen applied was in contact with the stigmas for periods of 4 to 10 hours. Five stages of sigma receptivity, based upon morphological characteristics and supplemented by observations on pollen germination and tube growth, are distinguished and described.