The existence of separate subtypes of type I vestibular hair cells according to morphological criteria in situ was investigated. Gerbils were anesthetized and perfused with mixed aldehydes. The crista ampullaris of the posterior canal was dissected, fixed in osmium, dehydrated, and embedded in epon. Five-micron sections were cut orthogonal to the long axis of each crista. Measurements were made on camera lucida drawings of individual cells located in the apical, middle, and basal 1/3 of the crista. Measurements for each hair cell included the circumference, greatest width of the body, length, width of the apical surface (cuticular plate region, P), width at narrowest portion of the neck (NW), neck width to plate ratio (NPR), length at a point 2 times NW from the apical surface (L2N). Type I hair cells were subgrouped into three classes (long -1, intermediate -i, and short -s) based on a subjective determination of neck length. Statistical comparisons were made between type I (n = 612) and type II (n = 74) hair cells and the type I subtypes (l, i, s). Statistically significant differences were found between type I and II hair cells for NPR, width, and length, but not perimeter. Thus, as in pigeons, NPR distinguishes type I and type II hair cells in the gerbil crista. While type I hair cells are wider and longer than type IIs, the circumference is the same, due to the restricted neck in type I hair cells. The L2N statistic separates three subtypes of type I hair cells.(ABSTRACT TRUNCATED AT 250 WORDS)