Knowledge of the distribution of species is fundamental to their conservation. Hence, it is important to understand the completeness of distribution inventories and the location of the main gaps. In an attempt to do this within an area of north-west England, this study builds a model which predicts bryophyte species-richness in 1220 tetrads (2 × 2 km squares) based upon their environmental characteristics, then compares predictions with current knowledge. The number of raindays (≥1 mm rain) each year was clearly the most important predictor of species-richness, followed by total nitrogen deposition and percentage cover of broad-leaved woodland. The main gaps in the current inventories are located and an assessment is made of their overall completeness. Suggestions for improvements in predicting bryophyte species-richness at tetrad resolution are provided.