P. vulgata L. were collected from high and low-water levels on five different beaches and the lengths of the shell and radula measured. In all five localities the mean shell-length was less and the mean radula-length greater at the higher level, but the difference was not consistently significant statistically. This complexity arose from the merging of two conflicting tendencies one, intralevel, a positive regression of shell-length and radula-length; the other, interlevel, a negative regression. Whilst the former is no doubt a growth phenomenon, the latter is probably an environmental effect[long dash]at the higher level exposure to desiccation and consequently, prolonged adherence to the substrate, causes a smaller shell base; reduced feeding time causes a longer radula. These factors may have resulted in confusion when comparing the values of the radula fractions of species of Patella. The intralevel regressions showed some variations; in three stations (all comparatively sheltered from wave action) they diverged from a point at the minimum population size and in two (exposed to wave action) they were parallel. The average slopes of the latter pair differed.