Ten 2-substituted 4-thiazolecarboxaldehyde hydrazones bearing 5-nitro-2-furyl, 5-nitro-2-thiazolyl, and 1-methyl-5-nitro-2-imidazolyl functions have been prepared and screened for activity against Trypanosoma cruzi infections in mice. The results permitted the ranking of these substituents in decreasing order of activity: 1-methyl-5-nitro-2-imidazolyl greater than 5-nitro-2-furyl greater than 5-nitro-2-thiazolyl, the last being inactive. Some structural features of the side chain necessary for optimum activity are discussed. The most active compound, 4-[[[2-(1-methyl-5-nitro-2-imidazolyl)-4-thiazolyl]methylene]amino]thiomorpholine 1,1-dioxide, compared favorably with the standard Nifurtimox against three recent clinical isolates of T. cruzi, including one with a high myocardial tissue infiltration.