By applying descriptive set theory to the Wagner's fine structure of regular ω-languages we get quite different proofs of his results and obtain new results. We give an automata-free description of the fine structure. We present also a simple property of a deterministic Muller automaton equivalent to the condition that the corresponding regular ω-language belongs to any given level of the fine structure. Our results and proofs demonstrate deep interconnections between descriptive set theory and the theory of ω-languages.