ABSTRACT ATHEORY is developed to predict the flow resistance in vegetated open channels. The solution technique is based on an analytical method originally proposed by Roberson for smooth conduits roughened with discrete submerged roughness elements. The theory uses small diameter cylinders to simulate vegetative roughness elements. Flow conditions may either partially or fully submerge the cylinders. Included in the analysis is a method to predict the effect of flexible vegetation on flow resistance. The model also predicts resistance effects of a smooth boundary or one roughened by dense concentrations of small elements such as soil particle aggregates. An initial comparison of flume measurements of resistance for small diameter cylinders is given. The analytical model provides, in addition to resistance factor, other flow parameters involved in analytical solutions of vegetated open channel flow.