Two species of Hygrohypnum are described as new from Mexico: H. robinsonii has falcate-secund, slenderly acuminate leaves with a short, double costa, small alar cells, and serrulate tips; H. arsenei has falcate-secund, slenderly acuminate leaves with a single costa, enlarged alar cells, and entire margins. In connection with studies directed toward the publication of a moss flora of Mexico supervised by A. J. Sharp, two undescribed species of Hygrohypnum have come to my notice. The specimens, collected by Harold Robinson in Oaxaca in 1963 and by Brother G. Arsene in Puebla in 1907 were filed in the U.S. National Herbarium at the Smithsonian Institution as Hygrohypnum luridum (Hedw.) Jenn. I am indebted to Harold Robinson for making them available for examination. Hygrohypnum robinsonii sp. nov. (Fig. 1-4) Plantae parvae, graciles, pallido-virides. Folia falcatosecunda, concava, 0.5-0.7 mm longitudine, oblongo-lanceolata, sensim acuminata, acuta vel obtuse acuta, apice serrulata, costis duobus, brevibus, cellulis superioribus lineari-flexuosis, alaribus paucis, parve subquadratis. Plants rather small and slender, in loose, palegreen mats. Leaves falcate-secund, concave, 0.5-0.7 mm long, oblong-lanceolate, gradually acuminate and acute or bluntly acute; margins erect or incurved, especially above, serrulate at the apex or in the upper 1/4; costa short and double; upper cells linear-flexuose; alar cells scarcely differentiated, a few near the margins small and subquadrate to shortoblong. MEXICO. Oaxaca, ravine near Km 104, Rte 175, Sierra Juirez, above Valle Nacional, on wet rock surface near waterfall, May 19, 1963, H. Robinson 10,168 (type, us). As compared with H. luridum, the plants are much smaller, with leaves only about one-third as long and much narrower in proportion to their length.