When milt from the ayu Plecoglossus altivelis altivelis （Suborder Salmonoidei） was diluted with seminal plasma or artificial seminal plasma （ASP） consisting of 115.2 mM NaCl+12.8 mM KCl+1.6 mM CaCl2+0.6 mM MgCl2+20 mM NaHCO3 buffered with 20 mM TAPS-NaOH at pH 8.0, few spermatozoa initiated motility. The percentage of motility increased up to 35.9% with decreasing concentrations of potassium ions in ASP. Spermatozoa exhibited a high percentage of motility when they were diluted with isotonic nonelectrolyte mannitol solution. Adding KCl or NaCl to the mannitol solution （total osmolality of each solution was adjusted at 300 mOsm/kg by decreasing the concentration of mannitol） decreased the motility in a dose-dependent manner （0-5 mM in KCl and 0-105 mM in NaCl, respectively）. Calcium ions could overcome the inhibition of motility by potassium and sodium ions. Increases in calcium concentration in ASP increased the percentage motility up to 94.5% at 20 mM. Acidification of ASP at pH 6.0 also induced a high percentage of motility （76.3%）. These results indicate that ayu spermatozoa motility in the seminal plasma is suppressed with various ionic compositions （high concentrations of potassium and sodium ions, and low concentrations of calcium and hydrogen ions）.