Results are reported from experiments carried out in three seasons in an environment with mild winters and early springs in which the date of sowing of winter oil-seed rape was varied from early September to early December. Consistently high seed yields were achieved in the late sowings and, in two of the three seasons, delaying sowing beyond early September resulted in significant yield increases. The pattern of dry-matter accumulation and changes in leaf area index were affected to a large extent by sowing date, but no significant effect on total dry-matter production at final harvest was found. Numbers of axillary inflorescences per plant and, in two of the experiments, numbers of fertile pods/m2, were reduced by late sowing, but large increases in numbers of seed per pod were found in each experiment. Mean weight per seed was reduced when sowing was delayed but there was no consistent effect on oil content. Differences were found between cultivars in the extent to which delayed sowing produced increased yields. It is suggested that early spring growth, delayed flowering and reduced reflexion of radiation during flowering were important factors leading to the high yields achieved by late sowings. No interactions were found between sowing date and plant population density for any yield component in one experiment.