[en] The study of concentrations of ¹³⁷Cs in the soil is of fundamental environmental importance, since the chemical similarity of cesium and potassium can cause the accumulation of these radionuclides in plants. This work presents the determination of ¹³⁷Cs in the soils of the permanent plots installed in the Ecological Station of Assis (EEA), the Carlos Botelho State Park (PECB), the Caetetus Ecological Station (EEC) and the Ilha do Cardoso State Park (PEIC), representative of the Cerrado and Atlantic Forest biomes in the State of São Paulo. The soil samples were quantified by High Resolution Gamma Spectrometry - EGAR, with HPGe detector. The activity concentrations of ¹³⁷Cs were below the detection limit of 1.01 Bq kg^-1 in Cerrado soils (EEA). In the soils of the Atlantic Forest, activity concentrations of ¹³⁷Cs ranging from 1.32 Bq kg^-1 to 4.07 Bq kg^-'1 were quantified for EEC (Semi-deciduous Seasonal Forest), from 1.30 Bq kg^-1 to 7.40 Bq kg^-1 for PEIC (Restinga Forest) and 1.31 Bq kg^-1 at 23.30 Bq kg^-'1 for PECB (Dense Ombrophylous Forest). Considering the higher activity concentration values of ¹³⁷Cs in PECB, more detailed studies will be needed to know the fact of ¹³⁷Cs in the Atlantic Forest