There is presently much discussion over factors regulating soil microbial processes in grazed grassland ecosystems. We examined the relative importance of dung and soil fauna, and their interactions on microbial biomass and activity in two contrasting upland grassland soils. We found that the presence of nematodes and Collembola was associated with significant (P <0.05) increases in microbial biomass in both soil types. The addition of dung to both soil types also increased (P <0.05) microbial biomass, however, the effect was less pronounced than observed with animals. The addition of dung, or the presence of animals had no significant effect on microbial activity. The addition of nematodes had no effect on numbers of culturable bacteria or colony-forming units (CFU) or fungi. The presence of nematodes and Collembola had no effect on nitrogen availability. The addition of dung significantly (P <0.01) decreased ammonium-N concentrations in experiment 1 only. Observations on the development of nematode communities in microcosms are provided. Our results were taken to suggest the feeding activities of soil animals have a relatively more important role than dung deposition in determining the size of the microbial biomass in upland grassland soils. However, dung deposition appears to have an important role in providing additional substrate for microbial growth and metabolism, hence altering soil nutrient availability.