The effects of 1.24 kg ha-1 ammonium sulfate, 5 g ae ha-1 of the isopropylamine salt of imazapyr, and air temperature of 18, 27, and 35 C at 40 and 100% relative humidity on the phytotoxicity of 35 g ae ha-1 of the ammonium salt of imazethapyr applied postemergence to pitted morningglory were evaluated. Both ammonium sulfate and imazapyr increased imazethapyr phytotoxicity to pitted morningglory. Ammonium sulfate enhanced imazethapyr phytotoxicity the most at 40% whereas imazapyr was most effective at 100% relative humidity. Imazethapyr phytotoxicity was enhanced more by the combination of ammonium sulfate and imazapyr than with either alone regardless of relative humidity. Variations in temperature had little effect on phytotoxicity.