The electromyographic (EMG) characteristics of masseter, temporalis and sternocleidomastoid (SCM) muscles during maximum voluntary teeth clench were assessed in 27 male and 35 female healthy young adults. Subjects were divided into two groups: (i) ‘complete’ Angle Class I (bilateral, symmetric canine and molar Class I relationships), and (ii) ‘partial’ Angle Class I (one to three canine/molar Class I relationships, the remaining relationships were Class II or Class III). On average, standardized muscular symmetry ranged 80·7–87·9%. During maximum voluntary teeth clench, average co-contraction of SCM muscle was 13·7–23·5% of its maximum contraction. On average, all torque coefficients (potential lateral displacing component) were >90%, while all antero-posterior coefficients (relative activities of masseter and temporalis muscles) were >85%. The average integrated areas of the masseter and temporalis EMG potentials over time ranged 87·4–106·8 μV/μV s%. Standardized contractile muscular activities did not differ between ‘complete’ and ‘partial’ Angle Class I, and between sexes (two-way analysis of variance). A trend toward a larger intragroup variability in EMG indices was observed in the subjects with ‘partial’ Angle Class I than in those with ‘complete’ Angle Class I (significant difference for the temporalis muscle symmetry, P = 0·013, analysis of variance). In conclusion, the presence of a complete or partial Angle occlusal Class I did not seem to influence the standardized contractile activities of masseter, temporalis and SCM muscles during a maximum voluntary clench. Subjects with a ‘complete’ Angle Class I were somewhat a more homogenous group than subjects with ‘partial’ Angle Class I.