The multiplication of Vibrio parahaemolyticus on flesh samples of tuna fish was investigated. No multiplication of the organism occurred on the sliced raw red muscle, whereas remarkably abundant multiplication were observed on the sliced raw dark muscle. We then investigated the effects of pH, temperature and sodium chloride concentration on the growth and survial of the organism on sliced raw red and dark muscles. The results obtained were as follows: (1) A greater loss of viability of the organism on sliced raw red muscle of tuna occurred at 30°C than at lower temperatures. (2) The pH of the red muscle was close to 6.0 and that of the dark muscle close to 6.8. (3) The sodium chloride concentration of the red muscle was 0.18% and that of the dark muscle 0.34%. (4) A greater loss of viability of the organism occurred in either 0.18% peptone or homogenate prepared by blending raw red muscle and 0.15% NaCl solution. (5) Abundant multiplication of the organism was observed in homogenates prepared by blending the raw red muscle in 0.2% NaCl solution or a higher concentration. From the above results, it seems likely that loss of viability of the organism on sliced raw red muscle of tuna was caused by the low sodium chloride concentration of the red muscle.