Populations of Palmer amaranth (Amaranthus palmeri) that have increased tolerance to glyphosate have been described. These populations are not controlled by glyphosate rates of four to ten times of that required to control susceptible biotypes. Laboratory studies were initiated to examine the mechanism of increased glyphosate tolerance. Absorption, translocation, and metabolism studies were conducted. Preliminary studies indicate no differences in absorption, but some differences in translocation between tolerant and resistant biotypes. The two biotypes do differ in the level of shikimate present after exposure to glyphosate. The resistant population does not have a shikimate buildup indicating an insensitive EPSP target site or other mechanism preventing glyphosate from getting to the target site.