The results of the study of a forest floor of Arenito Bauru Soil are summarized as follows: The organic matter content of A00 and A0 horizons was determined by the loss on ignition method. The determination of the total carbon by the dry combustion method and subsequent calculation of the organic matter by the factor 1.724 was tound to be inadequate, judging from the low results obtaneid. Organic matter content of the forset floor of Arenito Bauru Soil is from 50 to 100% of that found in the first 30 cm of the soil mass, indicating a rapid decomposition of the same in the soil profile. Although Waksman (15) showed that lignin is the organic compound most resistent to the attack by the soil microorganisms, it was found that working with a soil from a different climatic region, lignin decomposes as quickly as all the other soil organic fractions. Therefore, concerning the fraction anti on of organic matter and the resistence of the compounds thus obtained to the decomposition by microorganisms, the idea was advanced of an approximately equal rate of decomposition for all the fraction for this soil type with the exception of the nitrogen compounds and water extract compounds. The fractionation of organic nitrogen compounds indicates enrichment of protein content in the soil with depth. It was determined that the losses of non nitrogen constituents were higher than those of nitrogen compounds. The accumulation of the latter in the soil takes place in the form of protein and related compounds. The C/N. ratio falls from 20 - 30 in the L layer to 10 - 14 in the layer a rising in b and e layers, which shows a definite washing of non nitrogen organic compounds into these horizons.