The purpose of this work was to study mineralogically the clay fraction of the Piracicaba series (RANZANI et al. 9). These soils belong to the Great Soil Group Red-Yellow Podzolic, Piracicaba variety. Four profiles were collected within the areas where Piracicaba series occur; these were labeled profiles P1 through P4. The soil horizons were sampled individually in each profile from the surface down to the rock bed. The clay fraction was subdevided by centrifugation in coarse and fine clay with 0,2 micron as their limit of separation. The identification of clay minerals in the two fractions and a semiquantitative analysis was done by interpretation of the results obtained from chemical analysis (% of K2O and cation exchange capacity) and X-ray analysis (difratogram using a Geiger counter and films using the powder method). The mineralogical analysis of the solo clay fractions, relative to the nature and quantity of clay minerals, indicate the following: the content of illite in the coarse clay fraction is always higher than 10% and in certain horizons shows a content of 30% or even 40%. The montmorilonite and the 14 A minerals normally occur with values lower than 10%, Kaolinite is the mineral that occur in greater quantity in the two clay fractions with a content always higher than 40%.