The normal development of the anatomic relationships between the motoneurons of the tibialis anterior (TA) muscle and their innervated muscle fibers was studied in 1-, 6-, and 12-week-old and adult cats. The motoneurons of the anterior branch and the contralateral posterior branch of the TA nerve were retrogradely labeled with horseradish peroxidase. Within the TA motor nucleus, anterior branch motoneurons (63% of total) were located rostrally and posterior branch motoneurons (37% of total) were located more caudally. The distributions of soma diameters of labeled motoneurons were bimodal in all age groups, allowing a presumptive division into gamma (small) and alpha (large) motoneurons. The posterior branch contained 52% of the total gamma motoneurons but only 28% of the total alpha motoneurons. Within the TA muscle, the regions innervated by the anterior and posterior branches were clearly segregated as determined by glycogen depletion. Myofibrillar ATPase staining at pH 4.4 demonstrated that the posterior branch innervated a higher proportion (56%) of types I and IIA fibers than the anterior. Our results support the hypothesis that a topographic relationship exists between the locus of a motoneuron within its motor nucleus and the position of its innervated muscle fibers within the muscle. Since these topographic relationships apply to all age groups studied, the muscle volume innervated by each muscle nerve branch appears to represent a reproducible developmental unit with distinct anatomic, physiologic and possibly functional properties. This unit may be termed a muscle "compartment."