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The problem of how semiconvection affects the interiors of massive main-sequence stars is reconsidered from a hydrodynamical standpoint. In these stars, semiconvection occurs in a region of smoothly varying molecular weight which forms in the wake of the receding convective core. A local application of linear stability theory indicates that the semiconvective oscillations that grow most rapidly have thermal relaxation times comparable to the oscillation period and wavelengths of around $10^3$km, corresponding to spherical harmonic degrees of several thousand. Because they are so dissipative, such oscillations are not excited as globally coherent modes. The nonlinear evolution of the instability in the interior of a 30M$_\odot$ main-sequence star is examined by means of two-dimensional numerical simulations, and the effectiveness of the instability in transporting heat and $^4$He is evaluated.
