In this work, we fabricate two types of hierarchical microspheres, i.e., one coassembled from two fluorene-based oligomers (1 and 2) and one self-assembled from a fluorene-based oligomer (1), for ultrasensitive and selective detection of trace sulfur mustard (SM) vapor. On the basis of distinct fluorescence responses of 1-2 coassembled and individual 1 hierarchical microspheres that originate from differential noncovalent interactions between analytes and these sensors, SM vapor can be ultrasensitively detected (30 ppb) and easily discriminated from various sulfides and other potential interferents. Our work that utilizes differential noncovalent interactions to give sensitive and selective fluorescence response patterns represents a new detection approach for SM and other hazardous chemicals.