Tree holes that contain water are rare. Occasionally one will be found which is in effect a permanent pool. With reference to their contained water they may be grouped roughly as those in the tops of living stumps, those in crotches or in the tops of protruding limbs or knots, and those in the sides of trees or limbs. Holes so far studied, with few exceptions, have contained rain water, that is, have been free from contamination by surface ground water. This report covers observations on 26 tree holes. About 140 species of algae and protozoa were recovered from these holes and are listed.