Bulk ZnSe single crystals were grown by annealing a CVD grown polycrystalline ingot in a selenium atmosphere for two weeks at 1000°C. To identify the defect structures closely related to the observed grain growth, point defects in bulk ZnSe were subsequently investigated using the positron annihilation technique. A striking result has been obtained; the selenium interstitial type defects were dominantly observed in the crystals grown under a selenium ambient. The fraction of annihilations from the trap states at selenium interstitial type defects in ZnSe increased with increasing the selenium pressure in the annealing ampoule. These results indicated that selenium interstitial type defects enhanced the atomic migration in the crystal and strongly assisted the recrystallization by dislocation climb.