The intramolecular nonbonded 1,5-type S ... O interactions are recognized in the crystalline structures of the (acylimino)thiadiazoline derivatives (1-3) as angiotensin II receptor antagonists. The relative stability of the nonbonded 1,5-type S ... O interaction was investigated using the X-ray crystallographic analyses and the ab initio MO calculations (HF/3-21G*, 6-31G*, and 6-311+G**) of the simplified model compounds (6, 7, and 9). The concept of mimic-fused bicyclic heterocycles consisting of fairly stable nonbonded S ... O interaction seems to be an efficient approach toward the design and development of various drugs.