The influence of photoperiodicity on hatching of Loligo forbesi and Loligo vulgaris embryos was investigated under different experimental light-dark (LD) conditions. The transition from light to dark stimulated hatching and functions as a ‘Zeitgeber’ or synchronizer. Independent of the timing and duration of the dark period most embryos hatched soon after termination of the light period. Embryos which had developed in constant light, showed no hatching rhythm at all. If these embryos were exposed to a dark shock most embryos hatched soon after the onset of darkness. A twilight shock, in which the light was reduced by 50% (i.e. 50 μE s−1 m−2), could not stimulate hatching. Embryos which were kept from stage X on in an artificially controlled LD cycle, preferentially hatch in a period which coincides with the period at which darkness usually occurred when placed in constant illumination from stage XX onwards.