A mathematical model of the parasitic phase of the life-cycle of Ostertagia ostertagi in calves is described. The model is used in the re-analysis of previously published data from a long-term trickle infection experiment in which groups of calves were infected daily with graded doses of 3rd-stage (L3) larvae. The results of the analysis are consistent with the hypothesis that the observed changes in the intensity of infection in the calves were the result of a decline in the proportional establishment of ingested L3 larvae, and a rise in the death rate of the 5th-stage worms as the duration of exposure to infection increased. The proportion of ingested L3 larvae that become established in the mucosa can be described as an exponential decay function of the duration of the infection. Within the range of trickle intensities investigated, the function appears to vary independently of the level of exposure to infection. In contrast, the rate of mortality of the 5th-stage worms appears to be an increasing linear function of the total cumulative number of 3rd-stage larvae administered.