The chromosome complement of male and female budgerigars (Melopsittacus undulatus) was studied in primary kidney cell cultures. van Brink's description of the male complement was confirmed. The 26 largest chromosomes can unequivocally be placed in 13 pairs. In the female the fifth largest chromosome is single ; it is the Z-chromosome, again confirming van Brink. In addition in all females and none of the males a previously undescribed extra chromosome was found, in size between pairs 9 and 10 with arm ratio 1∶1.5 and negatively heteropycnotic at metaphase. It is identified as the W-chromosome.