Rotavirus strains from 964 fecal specimens collected from children at 11 U.S. hospital laboratories from November 1997 to March 1998 and from samples collected at 12 laboratories from November 1998 to March 1999 were typed for G and P proteins. Serotype G1 was the predominant serotype in 1997-1998 (88%), followed by G2 (6.2%), G9 (3.3%), and G3 (1.5%). This pattern was similar to that seen in 1998-1999: G1 (79%), G2 (15%), G9 (3.0%), G4 (1.6%), and G3 (0.3%). Novel P[9] strains were identified in both seasons, and analysis of a 364-nucleotide fragment from gene segment 4 of one of the strains demonstrated 97.3% nucleotide identity with the prototype P3[9],G3 strain, AU1, isolated in Japan. This is the first report of a human AU1-like strain in the United States. These results reinforce our initial findings that serotype G9 persists in the United States but has not become a predominant strain and that the common serotypes G1 to G4 account for almost 90% of strains in circulation. Other uncommon strains exist in the United States but may have been overlooked before because of their low prevalence and the use of inadequate diagnostic tools.