The authors have performed differential measurements of fluorescence lifetimes by using photomultipliers with matched single-photon response for simultaneous acquisition of the fluorescence and excitation. This 'SAFE' technique automatically corrects for any change in the optical pulse during the experiment, thus eliminating a common source of systematic error. Preliminary results obtained using a standard reference of PPO in ethanol give a lifetime of 1.63+or-0.02 ns which is in good agreement with that obtained using conventional single-channel fluorometry.