We present the results of a diffraction-limited imaging survey of three nearby M-supergiants using interferometric techniques at the 4.2-m William Herschel Telescope. Between 1992 January and 1993 December all three stars (alpha Ori, alpha Sco and alpha Her) showed unambiguous evidence at optical wavelengths for asymmetric structures close to, or on, the stellar surface, implying that such features must be a common phenomenon on M-supergiants. Typically these unresolved features, or `hotspots', numbered between one and three, contributed between 5 and 20 per cent of the total flux from the source, and had similar colours to the emission from the stellar disc. The observed time-scales for variations of the hotspots were in the range 3 to 9 months, and are thus consistent with a convective origin, as first suggested by Schwarzschild, and with their contributing towards the irregular variability seen in this class of star. The apparent ubiquity of such surface asymmetries implies that the circularly symmetric models often used to interpret high spatial resolution observations of these stars will need to be modified if systematic errors are not to be introduced.