Strontium and rubidium contents of 22 whole-rock specimens and 15 residual glasses from Crater Lake, Oregon, range from 650 to 725 ppm and 23 to 33 ppm, respectively, for the less differentiated hypersthene andesites and basaltic andesites; to 150 to 400 ppm and 50 to 80 ppm for the more salic whole-rock and residual glass specimens. The behavior of these elements largely reflects the separation of large amounts of phenocrystic plagioclase. Sr content closely follows that of calcium. More sodic plagioclase exhibits a greater absolute affinity and a slightly greater relative affinity for Sr than more calcic compositions. K/Rb decreases progressively from about 440 at a K2 O content of 1.2 weight percent to 330 at 3.4 percent K2 O as a result of separation of plagioclase with a K/Rb ratio several times that of the associated magmatic liquid. Two factors support the hypothesis that the late dacites and rhyodacites of Mount Mazama were derived by low-pressure crystal fractionation of intermediate magma(s) that are very similar in composition to the much more voluminous andesites of the volcano. They are (1) the coincidence of the whole-rock–residual liquid tie lines, with general variation trends defined by the combined whole-rock and residual liquid data; and (2) the similarity of the coherent interelement variations, with trends inferred from data on elemental distribution between coexisting phenocryst minerals and magmatic liquid. Removal of 45 to 65 percent crystalline material from andesitic magma with 25 to 30 ppm Rb is required to produce the more salic rocks containing 50 to 65 ppm Rb.