This article argues that the power of the early Mercian kings fromWulfhere to Aethelbald, c.650-750, rested partly on their ability toexploit the growing economy which characterised this period; and thatsuch exploitation provides a hitherto unacknowledged reason for therise of Mercia to supremacy. The argument rests on the Mercian rulers' control of two particular places, London and Droitwich, the first thecountry's foremost port, the second a major industrial site concernedwith the production of salt. After the growth of Mercian authorityover both places has been traced, it is suggested that taxation oftheir activities may have resulted in substantial profits, via tollsin both centres and, in Droitwich, via a further levy on themanufacture of salt. The history of taxation on salt is traced backfrom the early modern period in order to see what light laterpractices shed upon those of the early middle ages. The articleconcludes by suggesting that the Mercian rulers were fortunate inexercising power at a time when economic growth, partly church-led,was open to royal exploitation and that those rulers had a consciousappreciation of the advantages so to be gained.