A maximum likelihood analysis is performed of 20 nrITS sequences of representatives of Plagiochila sect. Arrectae and P. sect. Rutilantes. Plagiochila rufescens from Chile and P. bicuspidata from Costa Rica form a well supported monophyletic lineage within the P. sect. Rutilantes. A sequence of P. exigua (sect. Rutilantes) from the British Isles composes with P. caduciloba, P. cuneata and P. steyermarkii a clade that is clearly separated from the P. bicuspidata clade.
As a result of the molecular investigation, P. rufescens is excluded from the synonymy of P. exigua and placed in the synonymy of P. bicuspidata. Several years old herbarium specimens of P. bicuspidata from Costa Rica and Chile still exhibit a distinct odour of peppermint which is unknown from European P. exigua. Only vigorous plants of P. bicuspidata can be separated from P. exigua by morphological characters.