Morphological variation of three Kaua'i species of Asplenium in the Diellia alliance is evaluated based on recent field observations and herbarium specimens. Their taxonomy, nomenclature, and synonymy are discussed, and a lectotype is selected for Lindsaya knudsenii. Asplenium diellaciniatum is interpreted as having extremely variable frond morphology and dissection, whereas A. dielmannii and A. dielpallidum are morphologically much more uniform. Their conservation status and population sizes are reviewed.