Abstract A series of measurements have been carried out to derive values for the spectrum-averaged fission cross-section of 235U and 239Pu for 252Cf fission neutrons. Two nearly identical target foils were mounted on either side of a Cf source (10 7 neutron/sec) in a compensated beam geometry. Fission fragments passing through limited solid angle apertures were recorded from each foil by solid-state track-etch techniques. The Cf neutron source strength was calibrated in manganese bath relative to the standard source NBS-II. Values of 1.215 ± 0.022 barn for 235U and 1.790 ± 0.041 barn for 239Pu were obtained for the fission cross-sections, corresponding to a ratio value of 1.473 ± 0.041.