Stream automata (also called ω-automata) and ω-regular languages are of paramount importance in Computer Science and Logic. A coalgebraic treatment of these structures has not been given yet. We study a simple two-sorted setting where deterministic Muller automata can be cast as coalgebras, so that coalgebraic bisimilarity coincides with language equivalence. From this characterisation, we derive concise and natural decision procedures for complementation, union, intersection, and equivalence check.