A 13-month field and laboratory study was conducted at Santa Catalina Island, California, USA to determine some of the effects of water motion on the shallow water distribution of the sea urchin Centrostephanus coronatus Verrill. The depth distribution of another sea urchin species, Strongylocentrotus franciscanus (A. Agassiz), includes shallow areas of strong water motion (<2 m depth) from which C. coronatus is absent. Differences in attachment strength and morphology of the two species were investigated as possible mechanisms affecting the differences in depth distribution. Density of C. coronatus along subtidal transects generally increased with increased depth, ranging from 0 individuals at depths less than 2 m to 1.7/m2 at 16 m depth; the majority of the population occurred deeper than 4.2 m. Measurements of relative water motion, using clod cards, made near the shallowest depths of occurrence of C. coronatus indicate that these depths are characterized by high levels or sharp increases in water motion. Laboratory wave channel experiments conducted at Scripps Institution of Oceanography, La Jolla, California, indicate that C. coronatus are dislodged at lower wave amplitudes when exposed to sharp increases in amplitude, than when exposed to more gradual increases. These results suggest that areas of strong water motion such as noted along the field transects may directly or indirectly limit the distribution of C. coronatus. Evidence suggesting that the effects of water motion on C. coronatus are relatively direct was obtained from field experiments in which C. coronatus were transplanted to deep (13 m) and shallow (3 m) quadrats, and changes in the number of individuals were determined following exposure to calm (≥ Sea State 2; n = 2) as compared to rough (> Sea State 2; n = 6) conditions. The majority (7 to 9 out of 10) of the animals remained in the deep quadrats following calm and rough conditions, with 5 or 6 remaining in shallow quadrats following calm conditions. In contrast, 0 to 2 individuals remained in shallow quadrats following exposure to rough conditions. Additional results of wave channel experiments indicate that C. coronatus are dislodged at lower wave amplitudes than are S. franciscanus. The ability of C. coronatus to withstand dislodgement during exposure to strong water motion was estimated indirectly by comparing the number of podia used in attachment by C. coronatus and S. franciscanus, Strongylocentrotus franciscanus had significantly more (p < .001) podia/animal than did C. coronatus for the animals tested, ranging from 56 to 100 podia per pore pair row for C. coronatus (n = 15), and from 146 to 214 for S. franciscanus (n = 16). Differences in the number of podia used in attachment were not significant, however results suggest that S. franciscanus uses at least 3 to 4 times more attachment podia (32 to 51 per pore pair row; n = 9) than does C. coronatus (2 to 8 per pore pair row; n = 10). Although other factors such as differential predation may contribute to the observed field distributions, results of the present study strongly suggest that C. coronatus is not well adapted to conditions of strong water motion, and that limitations in attachment strength, relative to S. franciscanus, probably limit the shallow water distribution of this species.