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The coupling of structure-specific in vivo chemical modification to next-generation sequencing has revolutionized the study of RNA secondary structure in living cells. Nonetheless, current approaches are limited by the detection of structural information as reverse transcriptase truncation products, which creates biases, blurs signal from heterogeneous structures, and is restricted to abundant RNAs. Here we present dimethyl sulfate mutational profiling with sequencing (DMS-MaPseq), which encodes DMS modifications as mismatches using a thermostable group II intron reverse transcriptase (TGIRT). DMS-MaPseq yields a high signal-to-noise ratio, can report multiple structural features for each molecule, and allows genome-wide studies as well as focused investigations of low abundance RNAs. We apply DMS-MaPseq to Drosophila melanogaster ovaries—the first experimental analysis of RNA structure in an animal tissue—and demonstrate its utility in the discovery of a functional RNA structure involved in the non-canonical GUG translation initiation of the human FXR2 mRNA. Additionally, we use DMS-MaPseq to compare the in vivo structure of messages in their pre-mRNA and mature forms. These applications illustrate DMS-MaPseq’s capacity to dramatically expand our ability to monitor RNA structure in vivo.