Forty-two complete spindle poles of cat superficial lumbrical muscles were analyzed with particular regard to the length and the diameter of intrafusal fiber types. Poles were reconstructed from serial transverse sections of fresh-frozen muscles. The staining module, which was repeated throughout the whole muscle, comprised sections treated for glycogen detection and sections treated for detecting myofibrillar ATPase activity after preincubation at three different pH's (see METHODS). The identification of intrafusal fiber types was essentially based on the ATPase activity of the B region of the intrafusal fibers. Long-chain fibers, i.e., chain fibers that have at least one pole that extends by more than one millimeter beyond the end of the spindle capsule (6), were very commonly observed. Of 42 spindle poles analyzed, 30 (71%) contained at least one long-chain fiber (one in 17 spindle poles, 2 in 11 poles, and 3 in 2 poles). Of 246 poles of chain fibers, 45 (18%) were "long". In four spindles, in which both poles could be completely examined, 10 long-chain fibers were observed. In eight of these, only one pole was long; the opposite pole ended either intracapsularly or at a short distance outside the capsule. Since long-chain fiber poles, presently considered to be among the effectors of static skeletofusimotor (beta) axons, are present in a large proportion of muscle spindles of lumbrical muscles, it would be of particular interest to reevaluate the beta-supply of these muscles by physiological methods.