A recessive mutation (bir) of E. coli, at 77 min on the standard map, destroys ability of the bacteria to take up biotin or to grow except at high concentrations of biotin. In a biotin prototroph, the bir mutation causes overproduction and excretion of biotin, and derepression of at least one biosynthetic gene (bioD). Growth of the bir mutant is inhibited by avidin or high concentrations of dethiobiotin. Both effects are reversed by biotin.