A study was made of the horizon under isovolumetric weathering, made up of halloysite, of a ferrallitic soil in the humid zone of the State of Bahia, Brazil. After two treatments with concentrated hydrogen peroxide and combustion of the sample at 800°C the residual carbon was 0·3 per cent. Determination of the isotopic ratio 14C/12C indicated an age of 3550 ± 150 years. Three hypotheses can be considered to explain the presence of this carbon in the horizon under isovolumetric alteration at a depth of 6 m: (i) it may have been derived from organic compounds originating in the upper horizons of the soil; (ii) it may be due to residual organic substances of microbiological origin; (iii) it may be carbon from the crystalline lattice of the clay minerals. No definite position has been adopted in favour of any of these hypotheses.