The objective of this thesis was to examine factors that influence the population dynamics of A. suum infections in pigs. In particular, the role of exposure in determining the pattern of aggregation of the parasite was addressed. In Chapter 2 data from a series of experiments investigating the effect of maternal exposure on the infection of offspring with A. suum were used to examine changes in the worm burden distribution, and the relationship between the aggregation parameter, k, and the mean intensity of infection. Analysis by maximum likelihood demonstrated that the colostrum of previously infected sows caused the distribution of worms among their piglets to become less aggregated, the degree of which determined by the length of exposure of the sow. A linear relationship between the aggregation parameter and the mean intensity of infection was also found. In Chapter 3, further experimental data from DCEP was compiled and used to examine the effect of inoculation protocol on the worm burden distribution. It was shown that trickle inoculations mimicked natural infections well, whilst single inoculations had a lower prevalence and mean intensity and a higher degree of aggregation. Chapter 4 describes an experiment which was designed to investigate the effect of exposure on aggregation, using a trickle inoculation protocol. The design incorporated additional investigations into the effect of inoculation dose level and duration of infection. Host predisposition, the effect of experience on parasite fecundity, and the development of a pre-hepatic barrier to infection were also examined. The new experimental results contradicted those found in a natural infection / reinfection experiment, and opposed the hypothesis that experience of infection causes a reduction in the aggregation of the worm burden distribution. However, a significant predisposition was found, experience of infection was shown to reduce the parasite fecundity, and larvae were shown to continue to migrate during a trickle inoculation. The experimental results also demonstrated that coproprevalence changed through time as a function of dose level. The relationship between coproprevalence and dose was used to develop a dynamic model (chapter 5). The model was used to demonstrate that the force of infection in the natural reinfection experiment was likely to have been considerably greater than those used in the trickle inoculation experiment. Although initially the immune response is greater in animals experiencing a high force of infection, through time a high force of infection leads to greater unresponsiveness.