The composition and richness of the microfauna on lime trees was surveyed in relation to the distribution and cover of lichens in Belfast. Parameters used to help interpret the results included distance from the city centre and available data on air quality. The percentage epiphyte cover on the trunks of lime trees was significantly correlated with distance from the city centre whereas that on tree bases as not. In contrast, the number of microfaunal species revealed strong positive correlations with distance for both the bases and the trunks of trees. Most of this increase in microfaunal species richness towards rural areas was due to protistans which are thus proposed as useful bioindicators of air pollution. The total species richness of fauna showed slight negative correlation with smoke but not SO2 levels.