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The 13C/12C ratios of occluded carbon within opal phytoliths from the northern Great Plains show potential as a basis for paleoclimatic reconstruction. A significant correlation exists between the carbon isotopic composition of a host plant and that of the organic matter in its phytoliths. The 13C/12C ratios for phytoliths from surface layers of soils along climatic gradients reflect the current proportions of C3 and C4 plants. Variations in the δ13C values of phytoliths with soil depth are caused by a variety of processes: burial of soil surface by dust, bioturbation, and possible illuviation by percolating water. Also, contributions of phytoliths by dust and roots have unknown isotopic effects. The δ13C values of phytoliths from soils increase with decreasing 14C age, suggesting that the proportion of C4 plants in this region has increased during the Holocene. Phytoliths of apparent mid-Holocene age suggest exclusive dominance by C4 plants which agrees with paleoclimatic interpretations of an arid middle Holocene climate.