The resonance integral for thorium metal rods of different diameters has been determined by the activation method. The irradiations took place in the central channel of the reactor R1, where the energy dependence of the neutron flux had earlier been investigated with a fast chopper up to several kev. The absolute calibration was made with gold as a standard. The true resonance integral for gold was taken from the literature as 1500 ± 35 b. The experimental values for thorium were fitted to two alternative expressions with the following results: The measurements were made for S/M values in the range 0.14–0.87 cm2/g. The main contribution to the margin of error arises from the uncertainties in the cross sections used and in the correction for the departure of the neutron energy distribution from the 1/E form.