During a stay which I made this year in the mountains of the state of Colorado I gave some of my time to the study of the fresh-water Rhizopods, comparing them with those I had observed in various regions of Europe. I should like to give here the results of my comparisons. All the organisms which are treated of in this paper have been found in the neighborhood of Caribou, a small mining town north of Boulder, and about 10,000 feet above the level of the sea. At this altitude Rhizopods are still numerous, as Leidy has shown in his remarkable work on the fresh-water Rhizopods of North America. He found them abundant in the Uinta Mountains, Wyoming, at 10,000 feet — the highest altitude, I believe, at which these organisms have as yet been found. However, as will be seen later, my gatherings at 12,000 feet have been very productive, which is after all not to be wondered at, knowing the very great capacity of these organisms for resisting either cold or heat, or any other disturbing element. In fact, they can be expected to be found everywhere, provided there are mosses and humidity, and if I have not found them higher (except one species, Difflugia constricta, at 12,500 feet), it is only because the ground was unfavorable to the presence of quiet water, and that my investigations at these higher altitudes have been very few.