Information on muscle length and tension are essential for the central nervous system to determine the position of body and limbs in space. This is a necessary prerequisite for co-ordinated motor behaviour. This proprioceptive sense is mediated by specialised mechanoreceptors located within the muscle: the muscle spindle (responsive to changes in muscle length) and Golgi tendon organs (responsive to changes in muscle tension). Anatomical and physiological analysis have provided many insights into the properties of the proprioceptor muscle afferents that innervate these mechanoreceptors but the molecular mechanisms that underlie the mechanotransduction process remains largely unknown. Here we show that the PDZ – scaffold protein Whirlin is expressed in proprioceptor neurones in dorsal root ganglia and localises to their peripheral sensory endings. Whirlin mutations have previously been implicated in abnormalities of hair cell and photoreceptor sensory transduction, particularly Usher syndrome (1), raising the possibility that Whirlin also functions in the proprioceptive mechanotransduction process. Recording afferent nerve discharge from an in vitro mouse soleus muscle/nerve preparation, we find that the activation of spindle afferents by mechanical stretch is greatly diminished in whirlin (wi) mutant mice (2) compared to their wild-type (wt) littermates (33.6 ± 10.4 Hz, 10 v 106.2 ± 28.2 Hz, 6; mean ± SE, n; P<0.01, paired t-test). Application of exogenous glutamate (1 µM) restores afferent excitability in response to stretch (111.1 ± 43.0 Hz, NS v wt). Amiloride (10 µM, an inhibitor of DEG/ENaC channels) and PCCG – 13 (1 µM , blocking atypical metabotropic glutamate receptors in mechanosensory endings (3)) reduced the glutamate-mediated enhancement in wt animals (from 205.1 ± 74.2 Hz to 95.4 ± 38.0 Hz, 4 (amil) & from 268.7 ± 37.9 Hz to 207.5 ± 43.8 Hz, 6 (PCCG); both P<0.05) but have comparatively less effect on the stretch – evoked spike frequency in wi mutants (to 99.8 ± 27.4 Hz, 6 & 93.2 ± 29.5 Hz, 8; both NS v 1 µM glutamate). Together, these data indicate that essential components of proprioceptor mechanotransduction machinery are present but operate inefficiently in the absence of Whirlin function. Whirlin may serve to recruit and/or ensure the proper localisation of transduction molecules in proprioceptor sensory terminals.