The two lateral apses were rebuilt in the 19th century.

Situated ca. 100 m south of the cathedral, the baptistery is the best-preserved structure to survive from Merovingian Gaul, although its origins lie in the Gallo-Roman period. In its present form it consists of a rectangular core with a polygonal narthex on the northwest side, semicircular apses on the lateral sides and a polygonal apse set into a trapezoidal projection on the southeast. The baptistery is orientated southeast, but since its lateral sides are considerably shorter than the others, they are treated like façades and carry gables. The Merovingian reconstruction is not precisely dated, but it has been convincingly attributed to the episcopacy of Ansoald (674-696).