Gunningite, AnSO 4 .H 2 O, occurs as sparse white efflorescences on sphalerite in several mines in the Keno Hill area, Yukon. The new mineral is monoclinic and isostructural with the other members of the kieserite group. Synthetic gunningite has cell dimensions of a = 7.566, b = 7.586, c = 6.954Aa; beta = 115 degrees 56'; space group C 2h6 - A2/a. The unit cell contains 4 formula weights. Indexed x-ray powder data of synthetic and natural material are given. The strongest lines of the powder pattern are 3.40(s), 4.78 (ms), 3.05(m), 2.51 (m)Aa. Gunningite is very fine-grained, white in color and streak, and has a hardness of about 2 1/2. Specific gravity of synthetic material is 3.195 (pycnometer); 3.321 (calculated). Refractive indices are alpha '= 1.570 gamma '= 1.630. X-ray fluorescence analysis of a gunningite precipitate gave ZnO 40.8, MnO 3.1, CdO 1.0, FeO 0.6, total 45.5%, in good agreement with the theoretical value of 45.35% ZnO in gunningite. The chemical relationships of gunningite with other members of the kieserite group were studied using synthetic material. Considerable quantities of Mg, Fe, and Mn may be substituted for Zn without effect on the configuration of the X-ray powder pattern. The association of other sulfates with gunningite in the Keno Hill area is briefly discussed.