This paper discusses the relationships between operating conditions and the turbidity caused by grab dredger having closed type grab bucket.
　The turbidity measuring points are located down the stream. The turbidity is recorded continuously on recording papers. Twenty or thirteen turbidity meters, whose measurements are based upon the photo-scattering principle, are used.
　It is clarified that the turbidity is higher at the middle and lower spaces than at the upper space, and that the turbidity is very low several ten meters away from dreding point Consequently it is made clear that the turbidity is evaluated by the parameter of [grabbed weight per cycle] ×[hoisting speed].
　The proposition of diffussion model of turbidity caused by grab dredger is offered. And the diffusion equation of turbidity composed of settling particles is solved on several simple assumptions.