AbstractIn spindles of rat muscles, the particular class of intrafusal fiber that is unreactive over its entire striated length when incubated for alkali‐stable “myofibrillar” ATPase, exhibits pronounced regional differences in staining following incubation for acid‐stable ATPase. This variation along the contractile polar regions of the fiber can easily lead to misinterpretations of intrafusal fiber type, as well as the erroneous suggestion of two distinct populations of muscle spindles. Furthermore, such regional staining differences are strongly suggestive of variations in the contractile apparatus along certain intrafusal muscle