AbstractThe fine structure of intercellular substances and rounded cells of the incisor dental pulp of guinea pigs is described. The extracellular fibrils are of two kinds; collagen with typical cross‐striations, varying from 400–700 Å in diameter, and fine fibrils of 100–120 Å in diameter. In cross section the latter fibrils appeared to be composed of three or four smaller subunits of less than 50 Å in diameter. The collagen fibril in the dental pulp appear singly or in small bundles of about a dozen or more fibrils. The fine fibrils are often aggregated along the cell surface.The ground substances are finely granular to fibrillar and show localized clumping which is related to fibrillar elements. The ground substances form an incomplete covering of 200 Å or more in thickness surrounuding collagen fibrils. When the collagen fibrils make a bundle, the sheath of grouund substances around one fibril becomes fused with that of adjacent fibrils.Rounded cells including macrophages, lymphocytes and eosinophils are present in the pulp. The fine structure of these cells is similar to that found in the same types of cells previously observed in oth