1. Chiasma frequency has been studied in male mice; fifty diplotene and fifty-six diakinesis counts were recorded. 2. The sex chromosomes differ from the autosomes during earlier stages of prophase, preceding them in contraction and in appearance and movement of chiasmata. 3. The total cytogenetic map length of mouse chromosomes measures 1954 cM. The average length of a short chromosome is 76 cM., those of medium and long chromosomes 106 and 127 cM. respectively. The sex element is 130 cM. long. 4. The presence of bivalents with five chiasmata in diakinesis suggests that medium and long chromosomes may sometimes form six chiasmata at an earlier stage, though none were actually observed at diplotene. 5. Similar numbers of chiasmata are formed in all chromosomes per micron of their spermatogonial metaphase length. There are fewer chiasmata formed in long than in short chromosomes per unit of their pachytene length. 6. The frequency distribution of chiasmata depends on the genetic constitution of the animal. 7. Genetic constitution seems to have an effect on the movement of chiasmata; in CBA animals they move more quickly than in hybrids. 8. When several chiasmata are present in a chromosome they move more quickly in long than in short chromosomes. But when there are only two chiasmata they move more quickly in short than in long chromosomes, where they may consist of merged chiasmata.