Electromyogram (EMG) biotelemetry was used to assess activity patterns for adult free-swimming largemouth bassMicropterus salmoidesand smallmouth bassMicropterus dolomieu. We first conducted laboratory respirometry trials and found a strong association between EMG signal and swimming activity which indicated that EMG biotelemetry could be used to assess activity of wild fish. A field study confirmed that both species exhibit diurnal activity patterns. When EMG activity was compared with estimates of swimming activity from location tracking, elevated EMG activity was often recorded for apparently stationary fish. These observations suggested that fish activity at spatial and temporal scales too small for detection by location tracking may account for a significant proportion of daily activity. We argue that EMG biotelemetry, combined with location tracking, may be a versatile tool for application to a wide variety of problems in fisheries biology, including the study of physiological energetics and spatial and temporal habitat use.