A series of naphthalenyl 3H-1,2,3,5-oxathiadiazole 2-oxides was prepared and tested for antihyperglycemic activity in the db/db mouse, a model for type 2 (non-insulin dependent) diabetes mellitus. Substitution at the 1-, 5-, or 8-positions of the naphthalene ring with a halogen was found to be beneficial to antihyperglycemic activity. 4-[(5-Chloronaphthalen-2-yl)methyl]-3H-1,2,3,5-oxathiadiazole++ + 2-oxide (45), one of the most potent compounds in this series, was selected for further pharmacological evaluation.