Brick samples from Greek churches of known dates were analyzed for geomagnetic intensity using the Thellier technique. In this technique bricks of known date are analyzed to obtain measurements of the past intensity of the earth's magnetic field by thermal analysis of thermoremanent magnetism. Some comparative determinations using the Shaw technique were also done. Results indicate a strong latitudinal and/or temporal dependence around 1300 AD. Churches dating from the end of the 13th century in the southern region of the samples indicate a value some 15% higher than those of early 14th-century date in the northern region.