Cerebrospinal fluid from a child with meningitis was found by immunofluorescence microscopy and by culture to contain a heterogeneous population of Haemophilus influenzae. Most of the bacteria possessed type b capsules and formed iridescent colonies. However, a few lacked typical capsules, and produced noniridescent rough colonies; although nontypable by the capsular swelling reaction, these bacteria synthesized type b antigen which appeared as discrete embossments. Embossed variants arose spontaneously in 23 of 25 other freshly isolated strains of H influenzae type b also. Transformation tests showed that a transmissible change of the bacterial DNA had occurred. The frequency of this genetic variation suggests that some "nontypable" H Influenzae found by other investigators in pediatric specimens may prove to be typable by immunofluorescence.