Picea sitchensis has a haploid complement of twelve metacentric or submetacentric chromosomes which are similar to each other in size. Prominent secondary constrictions, which are probably the sites of nucleolus organisers, occur on five chromosomes (N.O.R.s). The DNA content of the haploid, unreplicated chromosome set is 9 × 109 nucleotide pairs and, in spite of earlier claims, probably does not vary significantly between different populations. B-chromosomes occur extensively in the southern half of the species range. They have little effect on growth rate but delay the time of ♂ and ♀ flowering. Meiosis starts and ends in spring, taking about twenty-two days to complete. The B-chromosomes do not pair at meiosis and appear to be distributed at random. There is marked accumulation of B-chromosomes when transmitted through the ♀ parent, possibly due to preferential segregation during meiosis. A balance between gain by accumulation and loss due to delay of flowering may account for the natural distribution of the B-chromosome.