Differential staining of sister chromatids in male mouse meiotic cells was achieved by continuous infusion with BrdU from the tail vein followed by Giemsa staining (the FPG technique of Perry and Wolff, 1974). Analysis of 341 bivalents including XY, and 21 univalents reveals that: (1) visible crossing over coincided exactly with the site of chiasmata; (2) no evidence was obtained in support of chiasma terminalisation; (3) an anomalous type of crossing over was found in the monochiasmatic bivalents, which could not be explained by the conventional hypothesis for crossing over; (4) some of the terminally associated bivalents might be achiasmatic, and univalents might have originated from such bivalents; (5) in XY bivalents, sister chromatid association was between lightly and darkly stained chromatids, suggesting a lack of crossing over and the existence of other genetic controls for this association; (6) during meiosis sister chromatid exchanges (SCEs) might be less frequent than crossovers.