We introduce and study a new class of automata able to consume and produce multisets; we call them Mealy multiset automata. We are interested in their algebraic and coalgebraic properties. After some useful properties of multisets, we present the notions of bisimula-tion, observability, and behavior for Mealy multiset automata. We give a characterization of the bisimulation between two Mealy multiset automata, and a result relating their general behavior to their sequential behavior. We describe an endofunctor of the category of Set such that a Mealy multiset automaton is a coalgebra of this functor. This functor preserves coproducts, coequalizers, and weak pullbacks. Moreover, the new defined bisimulation is an instance of a more general coalgebraic definition.
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