Aorta myosin contains two kinds of light chain, 20-kDa phosphorylatable light chain and 17-kDa essential light chain (LC17). Purified myosin from porcine aorta media showed 3 distinct light chain bands on polyacrylamide gel electrophoresis (PAGE) in the presence of urea (urea-PAGE). The mobilities of the faster two components did not change after incubation of the myosin with a myosin light chain kinase. Gel slices containing the faster two bands were separately subjected to PAGE in the presence of sodium dodecylsulfate. Both components showed identical mobility with that of LC17. The two components were designated as LC17a and LC17b in increasing order of mobility on urea-PAGE. They were isolated by DEAE-Toyopearl ion exchange column chromatography. The amino acid compositions of LC17a and LC17b were similar to each other, but the contents of Ser, Met, Ile, and His were distinctly different. These results suggest that the two components are isoforms. The ratio of the content of each isoform (LC17a: LC17b) in the purified porcine aorta myosin was 39:61, and essentially the same ratio was found with washed muscle homogenate of porcine aorta. Then washed aorta muscle homogenates of rabbit and rat were examined. Two bands having similar mobilities to those of porcine homogenate were also found in urea-PAGE. The ratios of the two components were 31:69 and 66:34, respectively, for rabbit and rat. Aorta smooth muscle thus may contain many types of isomyosin.