ON April 8, 1976 Mars occulted the bright star ɛ Geminorum (magnitude 3.2). Predictions1 were issued well in advance since such occurrences are very rare: indeed, it is estimated that Mars occults a star as bright as this about once in 500 yr. Accurate observations of the duration of the occultation were made from four sites on the Earth and these are analysed here for the shape of the Martian atmosphere at that height at which the intensity of the starlight diminished by half. The oblateness is found to be about twice as great as that of the surface of Mars.