G207 and NV1020 are two replication-competent, multimutant oncolytic herpes simplex viruses evaluated in the current studies for their anticancer effects in the treatment of gastric cancer. Deletion of both gamma(1)34.5 genes and inactivation of ICP6 (ribonucleotide reductase) allows G207 to selectively replicate within tumor cells. NV1020 is another attenuated recombinant herpes virus with deletions of the HSV joint region, with deletion of only one copy of the gamma(1)34.5 gene, and with the ICP6 gene intact. In vitro, both G207 and NV1020 effectively infected, replicated, and killed human gastric cancer cells, with NV1020 being more effective at lower concentrations of virus. In a murine xenograft model of peritoneally disseminated gastric cancer, both NV1020 and G207 reduced tumor burden when given intraperitoneally (i.p.) at higher doses. When viral doses were lowered or when advanced tumor was treated, i.p. NV1020 was superior to i.p. G207. In vitro viral replication and cytotoxicity predicted the in vivo antitumor response. Intravenous delivery of either G207 or NV1020 failed to reduce tumor burden, demonstrating the importance of regional therapy as treatment for compartmentalized malignancy. Both agents were safe for use in animals, and immunohistochemistry performed on mouse tissue revealed selective viral targeting of tumor. Oncolytic therapy using genetically engineered HSVs represents a promising strategy for peritoneal malignancies.