A new local magnitude scale for West Australian earthquakes has been derived from first principles. The scale is defined by: ML(WA) = log A + 1.137 log R + 0.000657R + 0.66 + Si where A is the estimated maximum Wood‐Anderson trace amplidue (in mm), R is the hypocentral distance in kilometres (1 <R<2000) and Si is the recording station correction. The scale is based on more than 700 data points recorded from earthquakes located throughout the State. Strong motion data are included. Magnitude control was based on Richter's (1958) definition and the Wood‐Anderson seismograph recordings from the Mundaring Geophysical Observatory. Previous magnitudes determined by Richter's attenuation function are low for R < 70 km and high for R > 150 km, relative to the new scale. For R up to ∼ 200 km, the scale is quite similar to that of Hutton and Boore (1987) for southern California and Greenhalgh and Singh (1986) for South Australia, but beyond 200 km attenuation is lower in Western Australia.