A method for block synchronisation in optical PPM is proposed. This method is an application of the synchronisation method for PN sequences normally used in spread spectrum communication systems. In this method, the phase of the synch signal is shifted little by little, with the synch decision process carried out on the receiving side until synchronisation is attained. The decision rule is based on the fact that the number of received pulses in each frame of the synch signal must be one and that the time when the pulse is received should not be in the pre-fixed prohibited frame slot, provided that synchronisation is established. This method's synchronisation characteristics are investigated, and it is shown that block synchronisation can be established sufficiently with only 12.5% of additional prohibited slots.