The accident at the Chernobyl nuclear reactor in the USSR introduced numerous radioactive nuclides into the atmosphere, including the noble gas xenon-133. EPA's Environmental Monitoring Systems Laboratory, Las Vegas, NV, detected xenon-133 from the Chernobyl accident in air samples from a monitoring network that consists of 15 stations located in Nevada, Utah, and California. The peak concentration of xenon-133 was found in weekly air samples collected during May 6-13, 1986. The network average concentration of xenon-133 was 41 pCi cu m during that time. A lower average was found in air samples collected in the following week. These concentrations are comparable to or less than that of natural radionuclides (such as radon) normally present in the atmosphere, and are much lower than the peak xenon-133 concentration measured in New York State following the accident at the Three Mile Island reactor.