The effect of abrupt changes in photoperiod on induction of metamorphosis in Atlantic halibut was investigated. Larvae were reared in a 3.7-m 3 tank on enriched brine shrimp ( Artemia) from first feeding at 265 day degrees (d°C) (44 days post hatch, DPH) until 66 days (DPH) under continuous illumination or to a standard length of 17 mm corresponding to the beginning of metamorphosis. Thereafter, the larvae were divided into two groups; one further exposed to continuous light (24L:0D), the other given a 12L:12D regime, with four replicates within each treatment. The experiment was terminated at 116 DPH. An abrupt decrease in photoperiod at commencement of metamorphosis briefly stimulated eye migration and slowed growth. Growth was not significantly different at the end of the experiment. The eye migration to standard length ratio was significantly higher under 12L:12D than in larvae under continuous light at 116 DPH. Continuous light accelerated the appearance of haemoglobin in the circulation, gave a higher frequency of normal pigmentation, and higher survival than did 12L:12D in metamorphosing halibut larvae. The sequence of measurable metamorphic events was generally neural change, growth and skeletal change, circulatory change and finally establishment of the pigmentation pattern. Heterochrony of metamorphic events was observed for eye migration and for the appearance of haemoglobin with respect to age but not size. The results indicate that a short period of decreased photoperiod may act as an initiator for some metamorphic events The underlying mechanism may be the action of photoperiod on melatonin and stage specific thyroid hormone interaction. The interaction between ontogeny, photoperiod and endocrinology should be further investigated.