The ambient logic is a modal logic that was proposed for the description of the structural and computational properties of distributed and mobile computation. The structural part of the ambient logic is, essentially, a logic of labelled trees, hence it turns out to be a good foundation for query languages for semistructured data, much in the same way as first-order logic is a fitting foundation for relational query languages. We define here a query language for semistructured data that is based on the ambient logic, and we outline an execution model for this language. The language turns out to be quite expressive. Its strong foundations and the equivalences that hold in the ambient logic are helpful in the definition of the language semantics and execution model.