The morphology of the inner ear was investigated in the mutant strain ha of medaka (Oryzias latipes). The ha is a recessive mutant and ha homozygotes are viable but show abnormal circular swimming behavior. In adult ha/ha medaka, more than one semicircular canals are absent. In the most abnormal cases, no canals are present at all and the membranous labyrinth of the inner ear exhibits a simple rugby-ball-like structure. In spite of the apparent absence of the canals, however, receptor endorgans of the canals (crista ampullaris) and the nerves innervating the cristae (ampullar nerves) are present. Otoliths and associated receptor epithelia (maculae) as well as octaval nerve branches innervating maculae are also present, except utricular otoliths that are absent or extremely small if present. Projections of the ampullar nerves were also investigated, because central connections of the nerves may be also abnormal. Tract-tracing studies, however, revealed similar central projection patterns of primary afferents in the mutant and wild-type brains. These results suggest that membranes of prospective semicircular canals fail to form tubular structures and fuse with the membranes of otolith organs in ha/ha medaka. These results also suggest that abnormal morphology of the semicircular canals as well as the utricular otolith underlies the abnormal swimming behavior of the ha/ha medaka, in spite of apparently normal central projections of the ampullar nerves.