During the summer of 1957 the writer became interested in water mites while collecting insects in an unnamed pond (see map in appendix) near Tomales, California. It was noted during that period that specimens of a giant water-bug, Lethocerus americanus Leidy (1847) had many larval mites attached to their ventral sides, and the lack of information concerning these animals and their life histories stimulated interest in investigating the problem detailed in this thesis. This work was begun in the summer of 1957 and has continued through the summer of 1959 and 1960. Due to the lack of information and lack of agreement by other investigators concerning the morphology of the adult Hydrachna miliaria Berlese (1888) [bilunata Marshall (1931)], the problem of this thesis is to redescribe it and to describe the stages of its life history. Stages of the life history of H. miliaria have not been described previously by other investigators.