Sporadic dimetrism in some specimens of two English Wenlock favositid coral species, Favosites multipora and Paleofavosites rugosus, is investigated by serial sectioning. The dimetric appearance is found to be periodically developed within a colony where it occurs and to be caused by fluctuating rates of corallite increase. Peak increase corresponds in a regular pattern to other periodic features in these corals. Zones of close-spaced tabulae, thickened corallite walls and well developed septal spines form bands of denser growth occurring with an annual periodicity and it is concluded that increase and therefore dimetrism is also seasonally controlled in these corals. Entraining factors and the cause of modifications in the relative timing of peak increase are discussed. Dimetrism in these corals is considered to be wholly of ecophenotypic origin and of no taxonomic significance.