Western dwarf mistletoe (Arceuthobium campylopodum Engelm. 1. campylopodum) is a parasite of ponderosa pine (Pinus ponderosa Laws.). The objectives of this investigation are: (a) to formulate a mathematical description of the process of dwarf mistletoe disease spread in a pine forest, (b) to use this description to predict the spread in a few cases of interest, and (c) from the result to make some general hypotheses concerning the process. The simulation is based on a young-growth, managed ponderosa pine stand, where the trees are evenly spaced (9 to 18 feet apart), are of uniform height (10 to 25 feet), and have a light to moderate infection level. The model consists of four major submodels: tree growth, mistletoe seed production, seed dispersal, and infection establishment. The tree growth submodel provides information concerning size, position, and number of susceptible branches. The seed production submodel relates the amount of inoculum present to plant age. The process of disease spread is partitioned into a series of sequentially operating events. The probabilities associated with the events from mistletoe seed production to seed interception by a susceptible branch are computed in the seed dispersal submodel. The probabilities of subsequent events leading to infection are in the infection establishment submodel. Each submodel provides information for the next one, forming an interlocking set. Seven cases are examined using the complete simulation model. These include three tree spacings (9, 13, and 18 feet) with two moderate levels of infection (2 and 4 plants per infected tree) simulated for five years and one with a heavy infection level (15 plants and 9 feet spacing) simulated for ten years. The results are examined to assess changes in (a) the probability of infection with respect to tree spacing within a hypothetical stand, branchlet height, infection level, and time, and (b) the expected number of new infections. The model shows that the probability of reinfection decreases as the crown volume around a given height becomes larger and the foliage becomes sparser. The probability of infection due to contagion is found to decrease by about half for an increase in stand spacing of five feet. In a stand with an initial infection rate of 0.60 and a spacing of 9 feet, the expected number of new infections per 100 trees at the end of the fifth year is found to be 283 plants where there is an initial level of 2 plants per infected tree and to be 644 plants where there is a level of 4 plants per infected tree. Based on examination of the behavior of the model, five hypotheses concerning the disease spread process are formulated. (1) Plants high in the crown of the pine trees are the most important ones with respect to disease spread. (2) Where infection levels are moderate (fewer than 5 infections per tree) and where spacing is greater than 8 feet, vertical spread is accomplished primarily by reinfection. (3) It is possible for a tree to "outgrow" its infections. (4) In stands with spacing distances greater than 8 feet and a sparse mistletoe population, new infections are more likely to occur as a result of reinfection than as a result of contagion. (5) Increasing the spacing between trees reduces the probability of mistletoe infection from both reinfection and contagion. These hypotheses have a practi[c]al importance to the management of young pine forests. They indicate that selective thinning should discriminate against trees with infections at greatest heights. Also, in young stands with moderate infection levels, the chances are favorable for the trees to outgrow their infections, if they are spaced such that growth conditions are optimum.