SUMMARY.An account is given of the microfauna found in 93 samples of mosses from 18 localities in the Province of Quebec, one from New Brunswick, one fromNova Scotia and one from the Adirondacks in the United States. Species ofRhizopoda, Heliozoa, Mastigophora and Ciliata are summarised and the presence of associated microfauna such as Rotifera, Nematoda and Tardigrada is notified. In all 108 species of Sarcodina, 42 of Mastigophora and 40 species of Ciliata have been encountered. The microfauna of each moss sample is set forth, the mosses being grouped for each locality.Comparisons are made of the microfauna of the species of Sphagnaceæ, Polytrichaceæ, Dicranaceæ, Bryacoæ and Hypnaceæ examined.A quantative enumerative study of the micropopulations of a number of the mosses was made and the results are summarised. Huge numbers of Protozoa in many mosses are disclosed.Giant strains of some Testacea are notified.Some survival data of Protozoa found in Canadian mosses are given.Comparison is made of the Protozoal fauna of some of the Canadian mosses with those of the same species of moss in the United States and Europe.Some general observations, mainly physiological and distributional, in connection with the microfauna of the Canadian mosses are prese