Two components of the transmission dynamics of Trichostrongylus tenuis in red grouse are examined and quantified, namely parasite transmission rate and density-dependent reductions in egg production. Age-intensity data for birds of known age suggest that the rate of parasite uptake increases during the first 6 mo of a bird's life and this increase reflects an increase in feeding rate with age and exhibits no signs of self-cure. Analysis of these age-intensity curves permits us to estimate the transmission rate of the free-living infective stages. Reinfection rates of adults treated to reduce parasite intensities were not significantly different from infection rates of naive immature grouse. Secondary infections continued to rise over a period of 18 mo and this suggests that there is no strong host-mediated response against the parasite. Any density-dependent reduction in parasite fecundity is probably very weak and would act through interspecific competition between parasites. Initial analysis of worm egg production in relation to the intensity of worm infection found weak evidence of density-dependent suppression of egg production at high worm intensities. However, a more rigorous analysis found that such a relationship suffered from Type I errors and was a consequence of the aggregated distribution of the parasites. Any density-dependent suppression of parasite egg production is too weak to be detected and would only occur at high worm intensities. The potential density-dependent reductions in fecundity on the population dynamics of T. tenuis and red grouse are examined using a mathematical model. The model suggests that the presence of density-dependent reductions in worm fecundity could produce significant reductions in the propensity of the grouse-nematode system to exhibit population cycles. The sustained cycles observed in the long-term dynamics of the grouse populations in the study area suggest that density-dependent reductions in worm fecundity and establishment are either absent or only operating at levels that are not detectable in field studies.