Sisal showing symptoms of parallel streak developed more slowly than symptomless plants, both leaf length and number of leaves unfurled being reduced. In streaked areas of the leaf, ground tissues surrounding the peripheral and internal fibre bundles collapsed; decortication of streaked leaves was difficult and fibre quality was adversely affected by reduction in length and discoloration. Parallel streak was seen only in vegetatively propagated Agave sisalana, in leaves lacking marginal spines. The condition was observed only where the mean minimum temperature fell below 16°C; symptoms appeared five months after the onset of a period of low temperature. Apparently every A. sisalana plant in Kenya is affected, but the condition is masked in coastal regions due to high mean minimum temperature. The disease seems not to be due to bacterial or fungal pathogens, or to nutritional factors, but may be caused by a virus, or a genetic ‘disease’ similar to June Yellows of strawberries.