Abstract Geomagnetic paleointensity determination have been made by the Thellier method using samples from 27 sites in Bulgaria. The samples include bricks, specimens taken from historic kilns, from prehistoric hearths and the sites of ancient fires. The ages of the samples, which range from about 4500 B.C. to the 19th century A.D., have been determined partly by the 14 C method and partly from archaeological evidence. The (residual NMR)-(induced TRM) diagrams tend to be less linear for the prehistoric samples either due to weathering or because the NRM is not a total TRM.