Differences in the size and distribution of chromosomal C-bands were investigated in 17 strains of Mus musculus. Each strain was found to have a characteristic C-band pattern. Closely related strains, substrains, and congenic strains, e.g., DDD and DRC, BALB/c and BALB/cAn, and CBA/J and CBA/H-T6, showed a similar C-band pattern, whereas striking dissimilarities in the C-band pattern was observed between the related strains, DDD and DHS, and SJL and SWJ, which were established independently from noninbred stocks of dd and Swiss mice, respectively. Virtually every autosome except for 1 and 2 showed C-band variation among the strains. The size of the C-band on 1,2, and the X appeared to be fairly constant among the strains. All chromosome pairs were homomorphic in each inbred animal; the between-strain difference in the C-band size was most easily observed in F1hybrids which showed the heteromorphic C-band patterns, expected from the parents. Measurements of the chromosome lengths of the heteromorphic pairs in a (C57BL/6J x DDD) F1 hybrid disclosed that homologues which had different size C-band had significantly different lengths, and homologues, one of which lacked C-band material, were the same length. Heteromorphism in the length of Y chromosome was also found; BALB/cAn had a Y smaller than the 19. These chromosome markers appear to be useful for characterizing each strain as well as for monitoring inbred stocks.