Anelastic properties of zinc single crystal of high purity (99.996 percent) have been studied, using bars vibrating transversely at audio frequencies in free-free modes. Internal friction, effective elasticity, and shape of resonance curve were determined against the strain amplitude. When the driving force exceeds a critical value, the internal friction increases, its maximum-amplitude-frequency decreases, and its resonance curve becomes asymmetrical. Furthermore, under this condition a discontinuous phenomenon, which is supposed to be closely correlated with slips occurring in the crystal, has been found. This discontinuous phenomenon was recorded under different stresses.