This work examined cell composition gradients in maturing tillers of switchgrass (Panicum virgatum L.) with the aim of developing baseline information on this important forage and biomass crop. Flowering tillers were collected from plants raised from seeds in a greenhouse and field, harvested at soil level and separated into internodes beginning with the node subtending the peduncle. Internodes were analyzed using microscopy, by fiber digestion, high-performance liquid chromatography and by gas chromatography-mass spectrometry to obtain anatomical and compositional data. Microscopy demonstrated the development and maturation of cortical fibers which eventually became confluent with the fiber sheath surrounding vascular bundles in the lower internodes. Detergent fiber analysis indicated increasing cellulose and lignin contents and decreases in cell solubles and hemicelluloses with increasing distance of the internodes from the top of the plant. Soluble phenolics were greatest in amounts and complexity in top internodes. The lower internodes contained greater levels of wall-bound phenolic acids, principally as 4-coumarate and ferulate.