To determine the Bordetella pertussis serotypes currently causing whooping cough in the United States, recently isolated cultures from different geographic areas were studied. Specific adsorbed antisera were prepared in our laboratory and used in both tube and slide agglutination tests. Among the 177 cultures isolated during 1966 and 1967 in seven states and one Canadian province, the predominant serotype was 1.3 (or 1.3.6), represented by 85% of the cultures. Fifteen cultures were serotype 1.2.3.4.6. The 376 cultures isolated in the Grand Rapids area during the last 30 years showed a changing pattern from serotype 1.2.3 during the early years to the currently prevailing 1.3 (or 1.3.6). During the 2-year study period, seven cultures of B. parapertussis were isolated in Michigan, and seven were received from other states. One culture of B. bronchiseptica, recovered from a child in Boston, was sent to us.