The organization of the DNA scquences in five species of Citeth~s (C. pygmaeus, C. J'ulvus, C. mq/or, C. parryi and C. undutatus) was determined from the reassociation kinetics of DNA fragments of various lengths and the size distribution of SI-nuclease-resistant duplexes of repetitive DNA. Only 15% of the genome of all the species studied consists of short unique and repeated sequences interspersed with a period less than 2 3 kb, whereas the major part of the genome is occupied by much more extensive sequences of two types, moderately long (3-15 kb) and very long (much more than 15 kb). On the basis of the number of moderately long single-copy sequences the species under study are divided into two groups, coinciding with their division into short-tailed and long-tailed ground squirrels: the short-tailed (C. pygmaeus, C. major and C. fulvus) possess far more such sequences (17 24%) than do the long-tailed ones (C. parrvi and C. undulatus) (1-7%). The same division is observed in the amount of very long single-copy sequences. The repeated DNA sequences of Citellus vary widely in size. i.e. from 70 up to some thousands of nucleotide pairs, sequences of more than 1200 nucleotide pairs being most common. In addition, part of the repetitions contain between 70 and 150 base pairs. About one-third of C. paro, i repeats (10% of the genome) are characterized by such very short sequences whereas their amount is much less in the other Citetlus species (1 4% of the genome).