We identified a Norwalk-like calicivirus (CV) whose genome likely was derived from naturally occurring recombination. This strain (Arg320) was detected by the EIA developed against recombinant Mexico virus (rMxV) capsids, but the viral RNA polymerase sequence was closer to Lordsdale virus, in a separate genetic cluster of Norwalk-like viruses. A 3.3 kb cDNA from the RNA polymerase region to the 3' end of the genome of Arg320 was cloned and sequenced. The sequence demonstrated that the capsid region of Arg320 shared 95% amino acid identity with MxV, but 68% identity with Lordsdale virus, while the RNA polymerase region shared 95% identity with Lordsdale virus, but 87% identity with MxV. Pair-wise sequence comparisons identified a potential recombination site at the polymerase/capsid junction. This is the first example of a naturally occurring recombinant in the CV family. Further studies to search for and characterize other strains may be necessary for understanding the genetic diversity of the family.