During their escape response, scallops swim, using jet propulsion created by rhythmically opening and closing their valves. Valve closure is powered by the large adductor muscle that acts against the hinge ligament. We compared in vivo force production during escape responses and in vitro isometric contractions measured on fiber bundles from the sea scallop Placopecten magellanicus. The in vivo recordings quantify force development during tonic and phasic contractions and can assess clapping frequency. In vivo responses were measured at les Iles-de-la-Madéleine (QC, Canada) after which the animals were flown to Quebec City where we measured in vitro contractile properties of isolated adductor muscle at 5°C and 10°C. Interindividual differences in force production were positively correlated between in vivo and in vitro measurements. However, peak twitch force and peak tetanic force (N cm^-2) of the isolated fiber bundles were lower than the maximum phasic force measured in vivo. This difference is likely to reflect damage to fiber bundles during isolation and the mechanical arrangement of the adductor muscle that leads the force measured at the edge of the valve during the acceleration of the upper valve to be greater than that measured near the muscle in vivo. Our comparison of in vitro and in vivo force production by the scallop adductor muscle underscores the advantages of our simple, minimally invasive, in vivo method for assessing the capacities of scallop muscle in natural situations.