Data on the distribution and timing of occurrence of cod ( Gadus morhua ) eggs and larvae around the British Isles is presented from ichthyoplankton surveys carried out from 1953-1990 in the Irish Sea, Bristol Channel, English Channel, and North Sea. The mean date of cod spawning ranged from the end of January in the eastern English Channel to late March in the Bristol Channel. Model simulations of seasonal cycles of chlorophyll in the surveyed areas are compared with data from the Continuous Plankton Recorder and used to test the hypothesis that the timing of spawning is closely coupled to the onset of primary production. The available evidence supports the hypothesis, but is not conclusive. An alternative hypothesis, that timing is a function of water temperature, is rejected. The location and timing of spawning may be adapted to ensure that larvae encounter adequate feeding conditions.