The timing of final maturation and competence for fertilization of oocytes from individuals ofNereis (Neanthes) virenswas investigated. In the first experiment, one group was decerebrated and the other was not. Both groups were placed into a long-day photoperiod (LD 16:8). The oocytes from the decerebrated animals became fertilizable and successfully developed to the larval stage in advance of the other group. In the second experiment one group was maintained in long-day and the other in short-day photoperiod (LD 8:16). The oocytes from the animals exposed to long-day photoperiod became fertilizable in advance of those maintained in short-day photoperiod. These results indicate an influence of external photoperiod on the final stages of oocyte maturation resulting in advanced fertilizability and successful larval development.