Segregation of thalamocortical afferent (TCA) fibers precedes barrel formation in rodent somatosensory cortex (Killackey and Leshin, 1975; Jeanmonod et al, 1981; Jensen and Killackey, 1987b; Senft, 1989; Erzurumlu and Jhaveri, 1990). Hypotheses about the arborization strategies followed by these ingrowing fibers have been generated from evaluation of labeled terminal fragments of mouse TCAs (Senft and Woolsey, 1991a). Those TCAs were of necessity truncated by the histological processing needed to observe them at high resolution. This fragmentation, along with biological variability, forces conclusions about single intact axons to be derived from populations of parts of arbors. To evaluate the hypotheses critically, we designed a computer program to quantify morphological aspects of labeled TCAs drawn with a camera lucida. We constructed algorithms to abstract, from fiber populations, properties minimally affected by truncation. Our program analyzes, and displays as histograms, fiber and branch densities and ori entations. To represent these features by additional graphical means, “average” ingrowing TCAs were generated, based on the accumulated statistics of the traced fiber fragments. Quantitative descriptions of TCA populations from postnatal day 1 (PND1) through PND7 are presented. These analyses show that fibers and their branches accumulate with age within the cortical plate (emergent layer IV), and to a lesser extent within developing layer VI. Simultaneously, the distributions of these afferents within cortical laminae transform from uniform to patchy in the plane of the cortex. Peaks exhibit the periodicity typical of mature barrels. Branches become more numerous focally as and where layer IV barrels emerge. Individually traced arbors show reduced total widths consistent with progressive pruning of branches extending into territories of inappropriate barrels, both in layer IV and deeper in the cortex.