Sites that are particularly rich in rare species are of significant conservation value and there is consequently a need to ensure that they are highlighted by methods which aim to select sites for potential safeguarding. This study measures the value for rare bryophytes of 434 sites, located in South Lancashire, north-west England (UK). Using cluster analysis, these are split into three groups, representing high, moderate and low rarity value sites. Sub-sets of sites of equal total area were then created for hypothetical safeguarding using two of the many site selection methods in operation, one being a traditional criteria-based approach and the other a maximum-coverage algorithmic procedure. Both methods failed to choose all sites of high value for rare bryophytes, while the latter included substantially more smaller sites of moderate and low value. It seems that a threshold type criterion assessed against a composite and continuous measure of rarity has the potential to improve the selection of sites of substantive wildlife value. This is discussed in the context of the networks of Local Wildlife Sites and Sites of Special Scientific Interest in the UK.