A study done by researchers at the University of Pittsburgh indicates that Roundup, an herbicide commonly used to control invasive plants, is toxic to frogs. As reported by SciScoop, the study was initially done to see if Roundup impacted the frogs by decreasing their food supply. The frog killer in Roundup is not the active ingredient, glyphosate, but the surfactant (polyethoxylated tallowamine) that helps the herbicide penetrate into the plants. You can also check out Dr. Rick Relea's home page here, and read the abstract for his recently published study here.
Clarification: Thanks to a member of the APHIS listserver for pointing out that it was not the glyphosate in the Roundup that is harmful to frogs, and that there are other glyphosate-containing herbicides that can be used to combat weeds and do not contain polyethoxylated tallowamine. I have corrected the paragraph above to reflect this, and shame on me for not reading the last two paragraphs of the story!