This article argues that the power of the early Mercian kings from Wulfhere to Aethelbald, c.650-750, rested partly on their ability to exploit the growing economy which characterised this period; and that such exploitation provides a hitherto unacknowledged reason for the rise of Mercia to supremacy. The argument rests on the Mercian rulers' control of two particular places, London and Droitwich, the first the country's foremost port, the second a major industrial site concerned with the production of salt. After the growth of Mercian authority over both places has been traced, it is suggested that taxation of their activities may have resulted in substantial profits, via tolls in both centres and, in Droitwich, via a further levy on the manufacture of salt. The history of taxation on salt is traced back from the early modern period in order to see what light later practices shed upon those of the early middle ages. The article concludes by suggesting that the Mercian rulers were fortunate in exercising power at a time when economic growth, partly church-led, was open to royal exploitation and that those rulers had a conscious appreciation of the advantages so to be gained.