A study of the meiotic behaviour and transmission of accessory chromosomes in a British population ofC. pyrenaicaDC. (C. alpinaWats.) has been carried out. The accessories are euchromatic but smaller than the A-chromosomes. They occur in 78% of the individuals sampled and are constant in number in all the cells of an individual. All the accessories are morphologically identical and in no cell has any association between the accessories and the A-chromosomes been observed. The accessory chromosomes behave normally at mitosis. During meiosis their behaviour differs from that of the A-chromosomes in that at metaphase I they are sometimes associated together to form accessory ‘bivalents’ or ‘multivalents’. At anaphase I the paired accessories segregate in the same way as the A-chromosomes. Unpaired accessories, however, do not divide and are generally incorporated in one of the daughter nuclei. At anaphase II the accessories divide and segregate regularly. There is a significant difference in the transmission of accessories through the male and female lines. The accessories are preferentially eliminated from the male line and retained in the female line. This provides a possible mechanism for their retention at a constant level and frequency in the population.