Several mesoionic 1,3,4-thiadiazoles, more properly termed anhydro-4-phenyl-5-substituted-2-thio-1-thia-3,4-diazolinium thiols, have been synthesized. Physical constants for the compounds have been determined and recorded. Preliminary screening against selected microorganisms indicated some useful inhibitory activity in vitro . The compounds were determined to have low intraperitoneal acute toxicity in the mouse.