This study examined the additive effects of two mutant genes, pallid (pa) and tilted head (th), on otolith formation in mice. Four strains of mice were bred:(1) a control strain, heterozygous for both pa and th.(2) a pallid strain, homozygous for pa,(3) a tilted head strain, homozygous for th, and (4) a double mutant strain, homozygous for both pa and th. The results were confirmed (1) behaviorally, by the animals' ability to swim, and (2) by histological examination of the inner ear. The findings suggest significant differences in mean otolith scores between all possible pairs of strains. As expected, the controls had normal otoliths and the pa/th strain the most severe otolith defects. Furthermore, there was a significant directional asymmetry in mean otolith scores between the utricle and saccule of the R and L ears for both pa and th strains. The results also showed a highly significant linear relationship between poor swimming ability and reduction of otoconia (r= 0.94, F= 92.14, p less than. 001).