We study strategy-proof rules for choosing between two alternatives. We consider the full preference domain which allows for indifference. In this framework, for strategy-proof rules, ontoness does not imply efficiency. We weaken the requirement of efficiency to ontoness and characterize the class of strategy-proof rules. We argue that the notion of efficiency is not desirable always. Further, we provide a simple description of the class of onto, anonymous and strategy-proof rules in this framework. The key feature of our characterization results brings out the role played by indifferent agents.