The purpose of this study was to clarify the functional difference of the masticatory muscles between 6 children with normal occlusions and 11 children with crowding dentitions, by investigating the interocclusal points and electromyographs from Hellman's dental stage IIA to IVA.<BR>1) As the stage progressed, it became clear in IIIC and IVA that the number of interocclusal points of the group with crowding dentitions became smaller than that of the normal occlusion group.<BR>2) From the results of the integrated value (the sum of the electric potential (S)S=TA+TP+M) of the electromyographs in the normal occlusion group, the ratio (M/S) of the electric potential in M to the total active potential became larger than that of TA/S, for the period from the IIIA to IIIB stage. In the group with crowding dentitions, however, M/S became larger than that of TA/S, for the period from the IIIB to IIIC stage.<BR>3) The length of all masticatory rhythms (druation, interval and cycle) in the group with crowding dentitions tended to decrease as the dental stage progressed. In the normal occlusion group, however, only the duraticn tended to decrease as the dental stagc progressed. All masticatory rhythms of children with crowding dentitions were longer than in the normal occlusion group for all the stages.<BR>4) The masticatory rhythms of the masseter muscle obtained by gum chewing tended to stabilize as the dental stage progressed in the both groups. The rhythms were more stable in the normal occlusion group than in the group with crowding dentitions for all the stages.<BR>5) The frequency of the occurrence of the silent period in the group with crowding dentitions was lower in comparison with that of the normal occlusion group in the IIIB, IIIC and IVA stage. The duration of the silent period in the group with crowding dentitions was longer than that of in the normal occlusion group.<BR>Based on the above-mentioned results, it is suggested that the developmental characteristics of the masticatory function in cases with crowding dentitions is retarded, compared with that in cases with the group with the normal occlusions.