The genotypes of both host and parasite may influence the outcome of parasitic infections, but few attempts have been made to quantify the effect of parasite genotype on macroparasite infections of socio-economic importance. We examined variation in particular traits during the infection in pigs with the parasitic nematode Ascaris suum. We infected 26 pigs with mixtures of equal proportions of embryonated eggs from 4 single female worms each with a unique mtDNA haplotype--the eggs from each female worm were a mixture of siblings and half-siblings. Pigs were necropsied on days 14, 17 and 28 following inoculation, which corresponded to time-points before, during and after the main immune responses against the nematode. A total of approximately 11,000 worms were recovered at necropsy. The location in the small intestine was recorded for all worms and the length and mtDNA haplotype were determined for about 4200 individual worms. There were significant differences in the distribution and abundance of the 4 individual haplotypes among individual pigs demonstrating strong interactions between parasite and host. We found significant differences in the abundance and position in the small intestine as well as the size of worms among haplotypes. We conclude that both parasite and host effects as well as the interplay between them play important roles in determining the characteristics and outcome of infection.