The interaction between the 2 nodular worm species in the pig, Oesophagostomum dentatum (O.d.) and Oesophagostomum quadrispinulatum (O.q.), was studied by comparing the development and distribution of the species following single or mixed infections. The faecal egg excretion levels were assessed at regular intervals from week 3 post-inoculation, and indicated a strong negative impact of the introduction of O.q. on the continued egg excretion of O.d. All pigs were killed 9 weeks after the first inoculations to determine the composition and location of the worm burdens in the large intestine. O.q. was found more anteriorly located in the intestine than O.d., thus confirming previous descriptions. When both species were present, the distribution of O.d. was moved further posteriorly and was more spread out than in single-species infections. There appeared to be no adverse effect of O.d. on the establishment and fecundity of O.q. However, the worm recoveries corroborated the egg excretion observations, namely reduced worm burdens of O.d. if O.q. was introduced, or if O.q. was already present. It is uncertain whether this effect is caused by differences in host reaction against the two species, or whether a more specific competition occurs between the two nodular worm species in pigs.