Estimates are given for the reliabilities of fifty-eight North American seismic stations, based on I.S.S. P-time residuals of Mexican earthquakes for 1941, 1942, 1943, and 1944. The ten most reliable stations were found to be Tinemaha, Riverside, Pasadena, Mount Wilson, Tucson, Lick, Weston, Palomar, Haiwee, and Seven Falls. The standard deviation of a single P arrival for the more reliable stations is ± 2.0 seconds or greater. A systematic difference in travel times between the California stations and eastern stations, and particularly between Ottawa and the Pasadena group, is strongly suggested. P arrivals at Ottawa are about one second faster than for the Pasadena group.