The major parvalbumins present in the iliofibularis muscle of Xenopus laevis were identified and the total parvalbumin content of different types of single fibers of this muscle was determined by polyacrylamide gel electrophoresis in the presence of sodium dodecyl sulphate (SDS). The criteria used in the identification of proteins as parvalbumins were: a relative molecular mass (Mr) between 10,000 and 14,000, an isoelectric point (pI) between 4.0 and 5.0, and a Ca2+-dependent mobility when run on a polyacrylamide gel in the absence of SDS. Four proteins were thus identified as parvalbumins: PA1, Mr 14,000, pI 4.90; PA2, Mr 11,000, pI 4.90; PA3, Mr 11,000, pI 4.95; and PA4, Mr 11,000, pI 4.25. An ultraviolet absorbance spectrum characteristic of parvalbumins was recorded for a purified preparation of these four proteins. Because the apparent Mr of rabbit parvalbumin in the gel system used was 14,000, whereas the true value is 12,100, it is not excluded that the Mr of component PA1 of 14,000 is an overestimation. The total parvalbumin content of muscles and single muscle fibers was determined using the supernatant obtained after centrifugation of tissue homogenates. Analysis of the protein pattern after electrophoresis in the presence of SDS of this fraction indicated that the Mr 14,000 and 11,000 protein bands contained virtually only parvalbumin. Quantification of the total parvalbumin content of relatively fast (type 1) and slow (type 2) contracting and relaxing single muscle fibers, using laser densitometric analysis of minigels, yielded mean values (mg protein/g wet wt., +/- S.D.) of 5.2 +/- 0.8 for nine type 1 fibers, and 1.9 +/- 1.0 for five type 2 fibers. Both fiber types contained about 2.5-times as much of the Mr 14,000 isoform relative to the combined Mr 11,000 isoforms.