During locomotion, animals rely on multiple sensory modalities to maintain stability. External cues may guide behavior, but they must be interpreted in the context of the animal’s own body movements. Mechanosensory cues that can resolve dynamic internal and environmental conditions, like those from vertebrate vestibular systems or other proprioceptors, are essential for guided movement. How do afferent proprioceptor neurons transform movement into a neural code? In flies, modified hindwings known as halteres detect forces produced by body rotations, and are essential for flight. The mechanisms by which haltere neurons transform forces resulting from three-dimensional body rotations into patterns of neural spikes are unknown, however. We use intracellular electrodes to record from haltere primary afferent neurons during a range of haltere motions. We find that spike timing activity of individual neurons changes with displacement, and propose a mechanism by which single neurons can encode three-dimensional haltere movements during flight.