Abstract Bacillus Calmette-Guérin (BCG) is the name given to a family of vaccines derived in 1921 by the in-vitro attenuation of Mycobacterium bovis. Subsequently, BCG seed lots were distributed globally, and both phenotypic and genotypic differences between strains have been described. As a step to understanding BCG diversity, we have reviewed the English and French historical record on 13 strains and DNA was extracted from these strains for restriction-fragment-length-polymorphism study. The written record suggests “early strains” obtained from the Pasteur Institute between 1924 and 1926 and “later strains” obtained after 1931. Molecular typing resulted in 3 clades, based on variability in IS6110-typing and the presence of mpt64 gene. With two exceptions, these clades correspond with strains obtained in 1924–26 (IS 6110−2 / mpt64+), 1926–31 (IS 6110−1 / mpt64+), and 1931 or later (IS 6110−1 / mpt64-) This analysis demonstrates that BCG has undergone genetic changes since 1921, consistent with ongoing in-vitro evolution.