This study investigated the biology and predatory behaviour of Asterias rubens (L ), as a potential predator ofcommmeroial scallopst Pecten maximus (L. ý and quejen scallops Chlamys opercularis (L. ). A population of A. rubens was monitored in a sub-littoral area by divers. The density of A. rubens varied little, annually, but lower densities were associated with an increase in sub-sea swell or with darkness. Parallel studies on activity were carried out in the field and in the laboratory. A high recovery rate but little net movement of tagged starfish was found within the area. This contrasted strongly with estimates of daily distance travelled and led to the suggestion of a "home range" in A. rubens. Different sized starfish spent equal time foraging and feeding, while "substrate" feeding was the most common form of feeding. Smallstarfish fed on sedentary items such as barnacles, while larger starfish fed on a greater variety of larger and more mobile prey e. g. prosobranchs* The importance in the diet of calorifically rich9rare food items, such as bivalves and carrion was noted, Five other asteroid species were commonly found and details of their biology noted. In particular# Stichastrella rosea fed, exclusivelyt on sedentary prey, while Marthas , terias glaciatis-and Henricia oculata showed broadly similar diets to large A. rubens. Sampling of two separate populations of A. rubens led to the development of reliable techniques for weighing and measuring starfisho The (armlength/armwidth) ratio (LW) was proposed as a condition factor for starfish. The trophic conditions of the two populations were indirectly assessed from bionomic factors. Mean weight for size, LW ratios and organ indices were combined with results from feeding experiments to demonstrate differences in trophic condition between the two populations. The differences were also reflected in bodywall, pyloric caecum and gonad indices, Improved trophic conditions, however, did not necessarily lead to a great Increase In reproductive output, The benthic populations of two sampling sites were examined to assess the relationship between scallop and starfish speciesq their relations to substrate type and the effect of fishing pressure. Little correlation between density of scallop species and A. rubens was found. An aggregated pattern of dispersion was found for (P. scallop species maximust C. -opercularis and C. tigerina)9 while A. rubens displayed a more random dispersion* The aggregated nature of scallop stocks was discussed in terms of spat settlement and active movement. The shell orientation and the numbers of recessed or byssally attached scallops was studied in relation to patterns of tidal flow and substrate type. The importance of physical availability of prey9 compared with density estimates was revealed In field and laboratory work. Observations on starfish behaviour and evidence of predation led to the suggestion that infaunal bivalves and Chlamys tigerina, were of greater significance in the diet of A. rubens than commercial scallop species.