The taxonomic status of the gastropods Patella aspera Röding and P. caerulea Linnaeus has been a contentious issue for some time due to substantial overlap in the distribution of the conchological traits used for their classification. Although a number of conchological traits, when taken altogether, allow a separation of the two species, confirmation of their specific status had to wait until clear-cut differences were found in non-conchological traits. In fact, support for the validity of the two species has been provided by radular, karyotypic, electrophoretic, and soft-part studies. This may be due, at least in part, to difficulties involved in analyzing shell traits with overlapping variation. In this article we apply techniques of statistical mixture analysis to data on the shell height in the successive growth stages of P. aspera and P. caerulea. Our purpose is to illustrate the suitability of mixture analysis to handle situations in which single traits with overlapping variation are used for species discrimination. The results indicate that shell height provides a reliable criterion for species identification and may help improve our understanding of single trait variation in these Patella species. We suggest that mixture analysis can he used as a convenient tool for the extension of this work to other conchological traits and other taxa.