Norwalk virus (NV) infection was recently found to be associated with gastroenteritis in U.S. military troops stationed in Saudi Arabia during the 1990 Desert Shield Operation. We identified a Norwalk-like virus in the stools of two military personnel with gastroenteritis by ELISA and IEM. By RT-PCR and sequence analysis, the nucleotide sequence of part of the polymerase region of each of these two "Desert Shield" strains (DSV275 and DSV395) was found to be 73% identical to the corresponding region of NV. In addition, one of the strains (DSV395), which underwent sequence analysis of approximately 2900 consecutive bases, had a genomic organization characteristic of the Caliciviridae. Comparison of the DSV395 amino acid sequence of the capsid region with that of three other viruses in the Norwalk group (Norwalk, Southampton, and Toronto viruses) showed amino acid identity of 47-68%. Consensus sequence analysis of these capsid proteins identified two regions of conserved amino acids that flanked an area of variable amino acids. In addition, the proteins corresponding to the capsid regions of DSV395 and NV were expressed in an in vitro translation system. Immunoprecipitation studies using the expressed capsid proteins and paired DSV395 or NV infection sera indicated the presence of shared antigenic sites between the capsid proteins of DSV395 and NV. However, hyperimmune sera specific for the self-assembled recombinant NV capsid protein did not react with DSV stool antigen in an ELISA, suggesting that there may also be unique antigenic sites not shared between DSV395 and NV.