Of a total of 220 stool specimens from children with acute diarrhea, mostly under the age of 3 years, collected in Paraguay between January 1999 and March 2000, 70 (31.8%) were found positive for rotaviruses (RV). Positive samples were characterized by electropherotyping and subgrouping. Sixty-one (87.1%) were classified as group A, subgroup II; one (1.4%) was classified as group A, subgroup I; six (8.6%) were group A, non-I non-II; and two (2.9%) were not tested. RV strains were G and P genotyped by reverse transcription-PCR. The following G types were detected: G4 (34.3%), G1 (21.4%), G2 (1.4%), and G9 (5.7%). Mixtures of human and animal genotypes were detected in 15 (21.4%) samples, and 11 samples (15.7%) were nontypeable. The following P types were detected: P[8] (48.6%), P[4] (1.4%), and P[1] (1.4%). A mixed type was found in 10% of samples, and an unexpectedly high percentage (38.6%) of nontypeable samples was found. The common human G- and P-type combinations P[8], G4 (15.7%) and P[8], G1 (14.2%) were detected. Mixed human and animal genotypes were observed as the following combinations: G4 + G5, G4 + G5 + G10, and G1 + G10 for G types and P[8]-P[1] for P types. The emerging G9 genotype was detected in four samples. These results show for the first time the diversity of RV circulating among children in Paraguay and contribute to the knowledge of this pathogen required to devise strategies to prevent diarrheal illness in this country. The finding of mixed genotypes may indicate interspecies transmission of RV between humans and animals.