Tooth replacement in the Lower Permian gymnarthrid microsaurs Euryodus and Cardiocephalus is described and analyzed in terms of the Zahnreihen model. Previous workers noted the existence of replacement in these genera but were unable to determine whether it was organized. Statistical analysis in the present paper shows that replacement was organized. After examining alternative organization patterns, it is concluded that both genera had simultaneous replacement. This replacement pattern was previously known only in the piranha, Serrasalmus. The evolution and function of gymnarthrid simultaneous replacement are discussed. It is pointed out that the salamander Necturus probably has simultaneous replacement, and that it may be present in placodont reptiles as well. This replacement pattern is probably more common than it now appears to be.--Journal abstract.