Abstract This model simulates on a daily basis cell differentiation in a radial file of fusiform cells; cell division in the cambial zone, cell enlargement in the secondary phloem, and enlargement and cell wall thickening in the secondary xylem. Input variables specified for each day of the growing season include the number of mother cells, rates of enlargement and cell wall thickening, and maximum radial cell dimensions. The rules for the behaviour of cells as they pass through successive phases of differentiation are based on available data and observations. Output variables for each day of the season include the position of each cell in the radial file, all cell radial diameters, and cell wall thickness in the xylem. The daily output may be compared with actual cell measurements from trees in the field to check the model and the values for input variables. Examples are given of simulating annual ring formation in a Pinus strobus tree growing under good conditions and a Pinus resinosa tree that suffered a summer drought. The cell dimensions of these model rings are almost identical to those in the rings being simulated. The daily output of the model permits following the daily differentiation of any one cell, which cannot be done by destructive sampling of trees. Examples are given showing how cells may be followed through the phase division to count the number of times each mother ceil redivides, and to study the frequency distribution mitoses in the cambial zone. Potential uses could be to convert environmental data into cell dimensions in an annual ring or, by summing radial files, to simulate cambial activity in whole trees.