The effective delayed-neutron fraction, βeff, has been measured in three cores of the fast zero power reactor FRO. The variance-to-mean method, in which the statistical fluctuations of the neutron density in the reactor are studied, was used. The experimental results were compared with theoretical values of βeff calculated with perturbation theory. The difference was at most 10%, which should be com-pared to the accuracy in the experimental values of ≈3%.