Several boll weevils were captured in pheromone traps in 2004 in an eradication zone near Tlahualilo, Durango, Mexico, an area where boll weevils had not been captured for about 10 years. It is possible that the boll weevils were from an endemic population that was too low in numbers to be detected, or that it developed from an influx of migrants. To identify the most likely source population of individual boll weevils captured in this area, we characterized microsatellite variation in boll weevils sampled in four other populations from northern Mexico and southern Texas. Both indirect measures of gene flow and individual assignment tests were employed to evaluate interpopulation movement. Analyses suggest that the boll weevils captured in the eradication zone near Tlahualilo were primarily from an endemic low-level population, but that this area also is receiving immigrants from a cotton growing region ~200 km to the north, near Rosales, Chihuahua. This study demonstrates that the use of microsatellites and population assignment techniques will be of great value in determining the most likely origins of boll weevils reintroduced to eradication zones in the U.S. and Mexico.