Immunohistochemical investigations with calretinin, a neuronal calcium binding protein, were made in the vestibular end organs of five guinea pigs and one chinchilla. A specific pattern of immunoreactivity of afferent nerve fibers was found. Immunostaining was restricted to thick fibers innervating the apex of the cristae or the striola of the utricular macula. A study of serial sections revealed that the stained afferents gave rise to calyx endings, but not to collaterals containing bouton endings. The results are consistent with the conclusion that, of the three classes of fibers defined by Fernández et al. (1988, 1990), only calyx units are calretinin immunoreactive. A count of the number of labelled fibers in the chinchilla crista suggests that the entire population of calyx units is immunoreactive. The conclusion is surprising since the physiology of calyx units does not differ qualitatively from that of other afferents (Baird et al. 1988; Goldberg et al. 1990). The presence of this protein in the calyx neurons may be related to specific post-synaptic functions of this type of afferents.