Among 1316 rotavirus specimens collected during strain surveillance in the United States from 1996 to 1999, most strains (95%) belonged to the common types (G1 to G4 and G9), while 5% were mixed infections of common serotypes, rare strains, or not completely typeable. In this report, 2 rare (P,G3) and 2 partially typeable (P,G?; P,G?) strains from that study were further characterized. The P strain was virtually indistinguishable by hybridization analysis in 10 of its 11 gene segments with recently isolated P2A,G9 strains (e.g., U.S.1205) from the United States, but had a distinct VP7 gene homologous (94.7% a.a. and 90.2% nt) to the cognate gene from P1B,G12 reference strain L26. Thus, this serotype P2A,G12 strain represents a previously unrecognized reassortant. Three P3 strains were homologous (97.8-98.2% aa) in the VP8 region of VP4 to the P3,G3 feline-like reference strain AU-1, but had a high level of genome homology to Italian bovine-like, P3,G3 and P3,G6 rotavirus strains. Two of the U.S. P3 strains were confirmed to be type G3 (97.2-98.2% VP7 aa homology with reference G3 strain AU-1), while the other was most similar to Italian bovine-like strain PA151 (P3,G6), sharing 99.0% a.a. homology in VP7. Cross-neutralization studies confirmed all serotype assignments and represented the first detection of these rotavirus serotypes in the United States. The NSP4 genes of all U.S. P3 strains and rotavirus PA151 were most closely related to the bovine and equine branch within the DS-1 lineage, consistent with an animal origin. These results demonstrate that rare strains with P and G serotypes distinct from those of experimental rotavirus vaccines circulate in the United States, making it important to understand whether current vaccine candidates protect against these strains.