FLOWERING LOCUS C (FLC) in Arabidopsis encodes a dosage dependent repressor of flowering. We isolated five FLC-related sequences from Brassica napus (BnFLC1-5). Expression of each of the five sequences in Arabidopsis delayed flowering significantly, with the delay in flowering time ranging from 3 weeks to more than 7 months, relative to the flowering time of 3 weeks in untransformed Ler. In the reciprocal experiment, expression of Arabidopsis FLC (AtFLC) in an early flowering B. napus cultivar delayed flowering by 2-6 weeks, confirming the requirement of this gene for floral repression. In B. napus, we show that late flowering and responsiveness to vernalization correlate with the level of BnFLC mRNA expression. The different BnFLC genes show differential expression in leaves, stems and shoot tips, but expression is not detectable in roots. Vernalization dramatically reduces the level of BnFLC transcript and restores early flowering in the winter cultivar Colombus. We conclude that BnFLC genes confer winter requirement in B. napus and account for the major vernalization-responsive flowering time differences in the different cultivars of B. napus in a manner analogous to that of AtFLC in Arabidopsis ecotypes.