The presence of usually single, elongated, compound crystalloids in nuclei of sieve elements is an outstanding characteristic of the phloem of Amsinckia douglasiana A. DC. (Boraginaceae). The crystalloid consists of two components forming alternating panels, or blocks, that extend through the entire length of the crystalloid and radiate from the centre where one of the components predominates. Three to seven panels for each component were recorded. One component consists of 4-sided tubules closely packed in highly ordered aggregates, the other of wider 6-sided tubules rather loosely arranged in paracrystalline aggregates. The crystalloid arises at the beginning of sieve element differentiation. Aggregates of 4-sided tubules appear first. In plants infected with the curly top virus, the crystalloids do not differ from those in non-infected controls in structure and conformation. But because the phloem in infected plants is hyperplastic, with most of the cells differentiating as sieve elements, the crystalloids are far more abundant in diseased than in healthy plants.