A single male mouse exhibiting almost total achiasmate meiosis with a concomitant increase in univalent frequency is reported. It was impossible to state whether this condition was caused by asynapsis or desynapsis. Histological examination of the testis showed an apparent block in spermatogenesis associated with a thickening of the tunica albugenica and the appearance of large, multinucleate cells in the tubule lumens. It is postulated that this abnormality might be caused by a single recessive gene mutation.