The effectiveness of earthworm species as bio-indicators of forest site quality was tested by a dataset of 180 plots in temperate lowland forest (Flanders, Belgium). In these plots, earthworm samples were taken together with vegetation surveys and additional site quality parameters. Vegetation surveys were used to calculate mean humidity, acidity and nitrification indices and to display sites in a humus quality-soil humidity-ecogram. In this ecogram, the earthworm biomass per species, per ecological category and in total was related with timber growth indices for ash (Fraxinus excelsior) and beech (Fagus sylvatica). The observed parallels and discrepancies are discussed.