THe repetition frequency of a highly repetitive DNA sequence has been measured in the genomes of Ch. thummi thummi and Ch, th, piger. This sequence is known to be involved in the evolutionary duplication of defined chromosomal segments leading to a significant increase in the genome size of Ch. th. thummi. Reassociation of this highly repetitive DNA sequence which has a repeat length of 120 base-pairs, with total Ch. th. thummi and Ch. th. piger DNA has shown that the repetition frequency in the Ch, th, thummi DNA is 5.5 fold higher than in Ch, th piger. In both genomes a 120 base-pair sequence is present as tandemly repeated sequence as shown by Southern analysis.