Karyotypes of in vitro primary and seventeenth passage cells of the B16 mouse melanoma were analyzed in detail and compared with those of normal adult male C57BL mouse cells. Two large, biarmed, metacentric markers, with individual distinguishing characteristics, were constantly present in the tumor cells. The longest telocentric chromosome was abnormally elongated compared to its counterpart in a normal mouse. A single minute was incorporated in the stemline karyotype of 41. In primary in vitro cultures, a relatively large number of cells contained 40 chromosomes and apparently lacked a free minute. Cells of seventeenth passage cultures, though maintaining the typical stemline number, showed separate trends toward the incorporation of an additional minute and toward polypoidy. Only one regularly identifiable “rabbit ear” (R.E.) chromosome occurred in the tumor cells, but an obvious secondary constriction in each arm of the longest metacentric, a probable isochromosome, suggested that a member of the normal mouse R.E. complement may have been involved in the formation of this marker. In an attempt to explain apparent discrepancies between previously published mouse idiograms, two different methods of calculation were used to prepare idiograms of normal C57BL karyotypes. Methods for preparation of a B16 stemline idiogram are illustrated and compared.