Though the ecological impacts of Roundup™ herbicide have been reported, little is known about its effects on invertebrates. The purpose of this study is to determine its toxicity to head regeneration in Dugesia (Girardia) tigrina, the brown planaria. Decapitated flatworms were chronically exposed to dilutions of commercially available Roundup™ herbicide (23°C and L: D cycle of 14: 10 h). Ten ppm of herbicide significantly impeded the head regeneration process, slowing the onset of one stage and aborting a later stage. Although 5 ppm did not adversely inhibit the stages of head regeneration, the negative phototropic behavioral response of the decapitated flatworms was delayed; 2 - 5 days after decapitation, 5 ppm caused a temporary but significant prolongation of the response. Ten ppm caused a significant and permanent slowing of the phototropic response during the entire period of head regeneration. Although aging the Roundup™ (10 ppm) over 4 months attenuated the effects on head regeneration, this herbicide still prolonged the negative phototropic response. Reports have praised the use of this herbicide for crop productivity; yet, it is important to evaluate the impact of Roundup™ on the ecosystem.