The size sequence of the molar teeth in three genera, including seven species, of the Cercopithecidae was examined on the basis of mesiodistal and buccolingual crown diameters, and rectangle measurements (mesiodistal d. X buccolingual d.). The determination of molar size order was estimated by the three methods of mean values, size sequences and reductive indices (M2/M1, M3/M1). The results obtained are summarized as follows: 1. The molar size sequence of the seven species. The pattern of sequence in the molars of the Cercopithecidae was divided into six types: (I) M1 less than M2 less than M3 or (M1M2) less than M3, (II) M1 less than (M2M3), (III) M1 less than M3 less than M2, (IV) (M1M2M3), (V) (M3M1) less than M2, and (VI) M3 less than M1 less than M2 or M3 less than (M1M2). 2. The upper molars. i) The most frequent pattern of mean values found was type II for all the species of the Macaca and Colobus. The two species of Cercopithecus showed predominantly the characteristics of types III, IV and V. ii) The molar size sequence individual data revealed that in the mesiodistal crown diameter types I and II were distributed frequently in the Macaca, types IV and V in the Colobus, and type VI in the Cercopithecus. The reduction of the third molar, therefore, seemed to be more pronounced in the Cercopithecus when compared with the other two genus, Macaca and Colobus, because type VI is the type with more size reduction in the third molar. In the case of the buccolingual crown diameters, the most common size sequences were found in types II and III for the Macaca and Colobus. In the Cercopithecus, however, type V exceeded other types in frequence. iii) The results of the reductive indices showed that the second molars in the Colobus were greatly reduced compared to the first, followed by those of the Cercopithecus and Macaca. In the third molar reduction, the Cercopithecus had a relatively small third molar compared to the first, following the Colobus and Macaca. 3. The lower molars. i) The pattern in the mean values were frequently found to be type I for the Macaca and Colobus, and types III and II for the Cercopithecus. ii) The size sequence in the frequency of the mesiodistal crown diameter was well presented in type I for the Macaca and Colobus, while types IV and V were distributed frequently for Cercopithecus in which the reduction of the third molar was noticeable.(ABSTRACT TRUNCATED AT 400 WORDS)