1,3,4-thiadiazolium-2-aminide, which is a class of mesoionic compounds, were tested against promastigote and amastigote forms of Leishmania amazonensis. Parasites were assayed with or without the drugs in axenic media, using pentamidine isethionate as a reference drug. The very promising results showed us the most active compounds were the 4'- and 3'-methoxy derivatives against promastigote forms, while the highest activity against the amastigote forms was obtained with the 4'-fluor and 3'-bromo derivatives.