Haemophilus influenzae strains of serotype a very rarely cause life-threatening infections. Examination of strains from the Gambia, West Africa, that caused septicemia, meningitis, or both revealed that a clone has emerged that carries a DNA deletion previously identified only in type b strains that is hypothesized to contribute to the special virulence of that serotype. This clone appears to have arisen by transfer of DNA between type a and type b strains, a transformation event that has happened more than once, as shown by the discovery Kenya, East Africa, of a clonally distinct type a strain bearing the identical deletion. The implications for the emergence of clinically important non-type b strains of H. influenzae are obvious.