The aim of this study was to evaluate the effects of ultra high pressure (UHP) on the adductor muscles of bay scallops (Aequipecten irradians) pertaining to certain physicochemical characteristics and microstructure. UHP treatment was applied at 200MPa and 400MPa, each application with one pulse and a holding time of 10min, or 2 pulses and a holding time of 5min per pulse, in a continuous or oscillation mode. Determination of textural parameters and L*, a* and b* colour parameters were carried out and compared with untreated scallops. Microstructure was observed using a scanning electronic microscopy (SEM) technique and, for microbial analyses, a total plate count was performed. SEM showed that UHP treatments induced a size reduction of the honeycomb structure of myofibres, giving a more compact appearance to the structure. Colour and compressibility enhanced by pressure treatment, however loss of hardness was observed. UHP treatments also reduced the initial load in total plate count of microorganisms to 10cfu/g. UHP is a competitive technology in food processing and ready for use on this high-priced seafood product.