This paper investigates the relationship between constitutional ideas and political action during the 1630s by comparing the privately expressed ideas of Sir Simonds D'Ewes regarding ship money with his conduct regarding the levy, especially while he was sheriff of Suffolk in 1639–40. The first section investigates the constitutionalist views expressed in D'Ewes's ‘autobiography’, unpublished during his lifetime, and their relationship to D'Ewes's attitude to the political role of the levy. The second section studies D'Ewes's conduct as sheriff, in which he gave almost no expression to constitutionalist ideas, and suggests that he struck a middle course between neglect and zeal, while finding means to oppose the levy through his connections at court. The third section seeks to establish the reasons for the inconsistencies between D'Ewes's privately expressed ideas and his public conduct, which may have lain in a belief that, in the prevailing political situation, criticism of the levy had, in order to be effective, to be expressed in terms acceptable to potentially sympathetic courtiers; D'Ewes adapted the tone of his comments on ship money to his audience in order to achieve political ends, but had also to act in ways which would make that tone convincing. Participation in the collection of ship money was therefore not inconsistent with opposition to it.