A potential mosquito control agent, xanthan gum powder, was applied to a still water surface in laboratory pans to determine its efficacy in killingAedes atropalpus(Coquillett) pupae. The ED50of xanthan gum was 6.18gm/m2,and the LT50was 14.2 min. When xanthan gum was applied at a rate of 22.47gm/m2, water depth had no effect on the level of pupal mortality. The xanthan surface layer prevented access by the pupae to the air-water interface. The layer did not significantly decrease the levels of dissolved oxygen in the test pans.