This paper extends the failure detector approach from crash-stop failures to muteness failures. Muteness failures are malicious failures in which a process stops sending algorithm messages, but might continue to send other messages, e.g., ``I am alive'' messages. The paper presents both the specification of a muteness failure detector, denoted ny <>M and an implementation of <>M in a partial synchrony model. We show that, modulo a simple modification, a consensus algorithm that has been designed in a crash-stop model with <>S, can be reused in the presence of muteness failures simply by replacing <>M with <>S.