The marshlands of coastal southern and eastern England had unusually high levels of mortality from the sixteenth to the nineteenth century. The unhealthiness of the environment aroused frequent comment during this period and it was attributed to an endemic disease known as "marsh fever" or "ague". Marsh parishes were perceived both as a danger to the local inhabitants and as a deterrent to potential settlers. This paper traces the geography and history of the "marsh fever" in England and shows that the disease was, in fact, malaria transmitted by anopheline mosquitoes. Malaria, once endemic in the coastal marshes of England, had a striking impact on local patterns of disease and death. Yet this study also suggests that the species of malaria endemic in England were vivax and malariae and not the tropical strains of P. falciparum. The paper outlines a number of ways in which "benign" forms of malaria, acting either directly or indirectly, as well as in conjunction with other factors, could have given rise to the unusually high death rates experienced in early modern marshland England. The discussion concludes with an examination of the reasons for the clinical disappearance of malaria during the nineteenth century, its reappearance after the First and Second World Wars and the problem of imported malaria in Britain today.