Numerical computations were carried out for transient natural convection in molten silicon (Pr = 0.016) in a relatively shallow crucible heated on the wetted surface and cooled by a crystal rod and at the free surface. At Ra = 10(5), six separate roll cells appeared with triangular isotherms. In each cell, a coaxial double spiral streak line was found to exist. At Ra = 10(6), the convection became oscillatory with eight peak temperatures in a circumferential direction. The oscillation was found to consist of simultaneous movement of the roll cells in both the vertical and circumferential directions.