We studied the source parameters of the 1957 Aleutian earthquake using tsunami waveform data recorded on tide gauges around the Pacific Ocean. Using a finite-difference computation, this tsunami can be numerically simulated. The tsunami records were inverted for the slip distribution on the rupture area. Results show that slip was concentrated in the western half of the aftershock zone with a maximum displacement of 7 m. The moment computed from the slip distribution is 88¿1020 nm, giving the 1957 earthquake a moment magnitude of Mw=8.6. The waveform inversion confirms that no slip occurred in the Unalaska Island area, making this area a possible seismic gap with a potential to rupture in a great earthquake and generate a Pacific-wide tsunami. ¿ American Geophysical Union 1993