[en] Two wide-angle photomultiplier systems were simultaneously operated over a baseline of nearly 30 km in a search for cosmic optical bursts of fractional microsecond time-scale. In 74 hours of overlapping observation, one event was recorded coincident to 1 ms as against the corresponding accidental coincidence value of 0.14 from Cerenkov light pulses from unrelated cosmic ray showers. The possible cosmic origin of this event, including that from primordial black-hole explosions, is discussed and the corresponding upper limits derived. (orig.)