The occultation of a star by Pluto on 1985 August 19 was observed from the Wise Observatory. The light curve of the occultation indicates that Pluto had an atmosphere as early as four years before perihelion passage. The derived scaleheight, together with recent estimates of the planetary radius and an estimate of the surface temperature, allows the determination of the mean molecular weight of the atmosphere. From this, the main atmospheric component could be N2 or CO, or even CH4.