Objectives: The aim of this study was to determine: (1) the frequencies of sixth cusptrait (c6); (2) the relationshipof sixth cusptrait between the lower second deciduous and first permanent molars. Materials and methods: With the longitudinal study design, the sample consisted 64 pairs of dental casts (32 boys, 32 girls, from 3 to 5 age and from 12 to 14 age).