An experimental investigation was made of the statistical fluctuations in neutron intensity which occur in a nuclear reactor. An ion chamber was exposed to reactor flux, and the fluctuations in its output current were analyzed in a tunable bandpass filter to get the frequency spectrum of these fluctuations, which has the shape of the square modulus of the transfer function. Results are presented of some measurements made on various low-power experimental reactors at Argonne National Laboratory. For reactors with prompt neutron lifetime between 15 and 70 μsec, the quantity l/β was determined within 5 per cent or better from a least squares fit to the transfer function thus measured.