Using careful techniques of fibre handling, mounting, and measurement of fibre dimensions and fibril angle, a study was made of the tensile strengths of kraft springwood and summerwood fibres of black and white spruce. Ciné microscopy at medium power was used to provide a record of each fibre during straining. Fibres in the entire population differed appreciably in tensile strength, but the difference appeared to originate from only two causes. First, fibres have different fibril angles, and because of the anisotropy of the cell wall, the fibre strength drops off rapidly with increasing fibril angle. The difference in strength between species and fibre type is completely controlled by fibril angle differences. Secondly, fibres frequently have defects - both natural, such as pits and induced, such as wrinkles and nodes. The ciné records reveal that these initiate failure and lower the fibre strength. For fibres with no obvious defects, strength is controlled by fibril angle and fits Hill's criterion of fracture for orthotropic materials.