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The B (9) chromosome of maize exhibits a very ordered type of instability at the second pollen mitosis, when nondisjunction may reach a level of 95%. Much less commonly the chromosome is unstable during early development of the kernel. Instability in the kernel produces recessive sectors in either the endosperm or the sporophyte, reflecting the absence of dominant markers carried by the B (9). The causes of B (9) loss in the endosperm and the sporophyte were investigated for the two observable classes of sectoring: fractional loss (single event) and multiple loss (mosaic pattern). The fractional class represents isochromosome formation by the B (9) (Carlson, 1970, 1971). Data presented here suggest that the isochromosome is a by-product of telocentric formation at the second pollen mitosis, and does not arise directly from the B (9) chromosome. The chromosomal basis for the mosaic pattern of B (9) loss is not completely known. However, one class of mosaic kernels displays a heritable instability of the B (9) chromosome which apparently results from ring chromosome formation by the B (9). The time of origin of the ring B (9) chromosome is prior to the second pollen mitosis, since the unstable chromosome generated in the male parent is transmitted to both the endosperm and the sporophyte. Finally, a genetic factor controlling B (9) stability in the developing endosperm has been found. A single plant (1818-1), crossed as a female parent to a B (9)-containing stock, induced a mosaic pattern of B (9) loss in the endosperm at a very high rate. The characteristics of this plant are being investigated.