Less inflectional categories are found in negated clauses than are found in affirmative clauses in Bumthang, a Tibeto-Burman language of Bhutan. It is common cross-linguistically for languages to make fewer contrasts in negative clauses than in affirmative ones. In this paper we focus on the less expected appearance of the ergative case in certain negated irrealis clauses, where the use of this case would be ungrammatical in the corresponding affirmative clauses. We sketch the aspectual and case-marking systems of the language, and then present data exemplifying the interaction of case, aspect and polarity, including the use of the ergative with arguments of monovalent verbs in negated irrealis clauses. We conclude by offering an account for the behaviour observed in terms of the pragmatics of implicature.