The composition and some physical properties of 96 varieties in the anthophyllite series are studied. Included is data on seven new anthophyllites from the pre-Beltian Cherry Creek rocks of Southwestern Montana. Chemical analyses, spectrographic analyses, optical properties, densities, and unit-cell structure are presented for the Montana varieties. Similar information is presented in part for 89 varieties described in the literature. X-ray structural studies are given for several doubtful varieties and some of these, such as amosite and high-manganese varieties from Bohemia, are shown to be monoclinic amphiboles. Main conclusions of the study are: 1. The anthophyllite and cummingtonite series are not isodimorphous. 2. The anthophyllite series is a 3-component one of limited isomorphism involving chiefly magnesium, iron, and aluminum. Iron (or Fe" and Mn") replaces magnesium from about 5% to about 50% of complete replacement calculated on the atomic basis. Aluminum replaces silicon nearly up to 2 atoms and the same is true of its replacement of Mg, Fe"; the formula H2Mg5Al2Si6Al2O24 is nearly fulfilled. 3. Magnanese is not important in the series. It is notable that many cummingtonites are high in manganese. Aluminum, high in many anthophyllites, is not a major constituent of the cummingtonite series. 4. Calcium is present in most anthophyllites, the average being about one-half of one per cent. About the same amount of sodium is present in many anthophyllites; potassium is present in negligible amounts in some anthophyllites and is absent in the rest. 5. The identification of anthophyllite is not certain unless based on x-ray methods. This is particularly true of asbestiform varieties.