The activity of the masseter and anterior and posterior temporal muscles was studied by electromyography (EMG) in the thirteen subjects with unilateral balancing side interferences and in twelve control subjects without such interferences. In both groups the EMG recordings were made during postural activity and various functions of the masticatory system and in the interference group they were repeated on two occasions following occlusal adjustment. The postural muscle activity was significantly lower in the interference than in the control group while there was no difference during maximal bite. The number of chewing cycles was somewhat longer in the interference than in the control group. During swallowing of water the maximal mean voltage amplitude was smaller in the interference than in the control group. The findings are discussed in relation to previous results and the conflicting ideas concerning occlusal factors in the aetiology of mandibular dysfunction.