The sternomastoid muscle of the rat can be subdivided into a "white" and "red" portion. The red portion predominantly contains type IIA fibers and the white portion type IIB fibers. Fiber bundles were dissected from the two parts of the muscle and their dynamic properties determined; both showed "fast-twitch" type characteristics. Small, but significant, differences were found in their isometric properties. Their force-velocity relationships were identical, although the maximum speed of sarcomere shortening (about 40 microns s-1) was significantly less than that of other fast-twitch rat muscles. Other differences in the respective force-length relations at different temperatures were noted. The desirability of pursuing these investigations on identified single fibers is emphasized.