The aim of this study was to determine the effect of tooth clenching and grinding on anterior temporalis electromyographic (EMG) activity. The sample included 30 healthy subjects with natural dentition and bilateral molar support, 15 with bilateral canine guidance and 15 with bilateral group function. An inclusion criterion was that subjects have to be free of signs and symptoms of any dysfunction of the masticatory system. Bipolar surface electrodes were located on the left and right anterior temporalis muscles. EMG activity was recorded during the following conditions: A. eccentric grinding from intercuspal position to the right lateral edge-to-edge contact position; B. clenching in right edge-to-edge lateral contact position; and C. concentric grinding from right lateral edge-to-edge contact position to intercuspal position. On the working side, EMG activity was not significantly different between conditions in both occlusal schemes. On the nonworking side, EMG activity was significantly higher during condition C than condition B in both occlusal schemes (mixed model with unstructured covariance matrix). When comparing by side, EMG activity was significantly higher during conditions A and B on the working side than on the nonworking side. However, there was no significant difference during condition C. EMG activity was significantly lower with canine guidance than group function on the working side (in all conditions) as well as during concentric grinding on the nonworking side. These results could explain muscular symptoms in the anterior temporalis muscles if the subject is experiencing parafunctional habits either while awake and/or sleep that exceed the individual's adaptation capability.