A cytoarchitectonic parcellation has been made of the cortex of the insula and of the adjoining parts of the temporal and parietal lobes in rhesus and squirrel monkeys. In conjunction with this, the intracortical distribution of the thalamo-cortical fibers has been studied by the autoradiographic tracing technique. There is a systematic change in the density, laminar distribution and general character of the intracortical thalamic afferent plexus which seems to follow, in particular, the progressive differentiation of cortical layering that occurs in moving from insular through granular to homotypical cortex. In the dysgranular and granular insular areas in which cortical lamination is indistinct, the thalamic plexus as demonstrated autoradiographically is sparse and extends through much of layers III and IV. In the granular cortex (areas 3b and AI), the thalamic plexus is densest and coarsest; it fills all of layers IV and IIIB and extends into layer IIIA. In the "second" and "third" sensory areas, such as the second somatic sensory and many of the auditory fields, the density of the plexus and its coarseness diminish slightly and the deeper half of layer IV becomes free of terminals. In the homotypical cortex, the plexus becomes sparser, finer and strictly confined to layer IIIB. In many areas there are additional indications of thalamic terminations in deeper layers. Where layers V and VI are not divided into sublaminae (e.g.,in areas 3b and AI) there is labeling of the superficial half of layer VI. Where layers V and VI become subdivided in the homotypical cortex, the auditory and adjacent fields were only observed in cases in which the magnocellular nucleus of the medial geniculate body was involved by the injection of isotope. The boundaries of the cortical projection fields of individual thalamic nuclei, as determined autoradiographically, are remarkably sharp and invariably coincide with a sharp architectonic boundary or with a zone of maximal cytoarchitectonic change. Zones of apparent architectonic transition never showed overlap of thalamic afferents emanating from more than one nucleus. These results raise for discussion the significance of architectonic structure in relation to cortical connectivity and have a bearing upon those studies that have attempted to relate the terminals of thalamic afferents to particular classes of cortical neuron.