A model simulating Dictyocaulus viviparus infection in calves is described. The present paper only deals with the parasitic phase of the life-cycle. Descriptions are given for establishment, development rate of juvenile stages, mortality rates of both juvenile and adult stages, and fecundity. Literature data were used to to develop parameter functions and to estimate initial values for constants. Development of acquired immunity, defined as the proportional ability of the host to reduce the number of parasite individuals in some stage or about to move into a next stage, against establishment (protection) or affecting mortality rates of juvenile or adult parasite stages has been included. The effect of immunity on one parameter or process is viewed as distinctly separate from the effect on another. Preliminary comparisons between model prediction and observations gives encouraging results, indicating that the model simulates experimental D. viviparus infection in calves reasonably well. Some quantitative discrepancies between prediction and observation make clear however, that not all parts of the model are accurate. Further experimentation is needed to re-evaluate current model description and to improve model simulation.