A taper equation and associated tables are presented for red alder (Alnus rubra Bong.) trees grown in plantations. The data were gathered from variable-density experimental plantations throughout the Pacific Northwest. Diameter inside bark along the stem was fitted to a variable exponent model form by using generalized nonlinear least squares and a first-order continuous autoregressive process. A number of parameterizations of the exponent were examined in a preliminary analysis, and the most appropriate form was determined. This was achieved by examining alternative models across geographic locations and silvicultural treatments on the basis of their ability to behave well outside the range of the modeling data by using an independent evaluation data set from across the region and a model validation procedure. Incorporating three easily measured tree variables-diameter at breast height, total tree height, and crown ratio-provided the best fit among location and treatment. This taper equation can be used to estimate diameter inside bark anywhere along the stem, inside bark volume of the entire stem to any top height diameter, and merchantable height and volume between any two points along the stem (i.e., individual log volumes). The flexibility of the model allows for accurate volume predictions across a range of operational stand conditions and management activities and is therefore an improvement over previously published red alder volume and taper equations.