This paper describes the results of the field test on the dredging effects of water-jets and teeth equipped with the trailing suction head of the Tokushun-maru No.1.
　The similar tendency in the relationship between the ship's speed and the solid concentration of dredged mixture was recognized for both tests with and without water-jets, and tests with and without teeth. The maximum solid concentration was identified at the ship's speed of between 3 and 4 knots. It was revealed that water-jets were always effective for both cases with and without teeth, but that the ratio of excess effect would decrease with the increase of theship's speed, and that little could be expected for the speed at over 4 knots.
　Within this test conditions, teeth were proved unbenefitable because the head with teeth showed comparative decrease in solid concentrations. This may be partly because in spite of the fact that they are useful for digging the sea bed they apt to make clearance between the sea bed and the grate of head resulting in deterioration of scouring effects due to the decrease of the flow-in velocity of mixture into the head.