1. The strength of individual filamentary crystals subjected to tension is close to the theoretical. Tests in bending and in fatigue corroborate the high strength features of “whiskers.” 2. The strength of filamentary crystals decreases with an increase in diameter and in length. It may justifiably be concluded that this decrease is due to the combined action of defects of different origin. 3. The whiskers of copper form along the axes of three basic orientations: [100], [110], and [111]. In the case of [111], the cross-section is hexagonal; in the case of [100] it is square. Whiskers more than 10μ in diameter develop a block structure and are markedly less strong.