Analysis of the organization of nucleotide sequences in mouse genome is carried out on total DNA at different fragment size, reannealed to intermediate value of Cot, by Ag+--Cs2SO4 density gradient centrifugation.--According to nuclease S-1 resistance and kinetic renaturation curves mouse genome appears to be made up of non-repetitive DNA (76% of total DNA), middle repetitive DNA (average repetition frequency 2X10(4) copies, 15% of total DNA), highly repetitive DNA (8% of total DNA) and fold-back DNA (renatured density 1.701 g/ml, 1% of total DNA).--Non-repetitive sequences are intercalated with short middle repetitive sequences. One third of non-repetitive sequences is longer than 4500 nucleotides, another third is long between 1800 and 4500 nucleotides, and the remainder is shorter than 1800 nucleotides.--Middle repetitive sequences are transcribed in vivo. The majority of the transcribed repeated sequences appears to be not linked to the bulk of non-repeated sequences at a DNA size of 1800 nucleotides.--The organization of mouse genome analyzed by Ag+--Cs2SO4 density gradient of reannealed DNA appears to be substantially different than that previously observed in human genome using the same technique.