In contrast to those of corn, the ring chromosomes of Drosophila melanogaster are, with one exception, relatively stable. This exceptional Drosophila ring, called wVC, is derived from a stable ring-X chromosome, and its instability is manifested through the frequent production of gynandromorphs, XO males, and dominant lethals among prospective wuc/rod progeny. It has been proposed that these manifestations of wvC instability are a consequence of anaphase bridge formation by the wvc chromosome, whereby gynandromorphs and XO males arise from bridge loss, and dominant lethals owe their origin to bridge breakage (HINTON 1955). Since the work of RICCLINTOCK (1938) and SCHWARTZ (1953) suggests that sister-strand exchange is responsible for anaphase bridge formation by the ring chromosomes of corn, it appears that the same process might be the cause of wVc instability. If this is true, the manifestations of instability should disappear upon conversion of the unstable wvc ring chromosome to a rod chromosome because ring structure is a prerequisite for anaphase bridge formation as a consequence of sister-strand exchange. Studies on rod-wUC chromosomes derived from unstable ring-wvc chromosomes, as will be described in this paper, show that these rods are unstable. Although the manifestations of instability by wuc rods are qualitatively different from those of the unstable ring-wvc chromosome, they nevertheless are capable of interpretation in terms of anaphase bridge formation.