Turnip yellow mosaic virus (TYMV) induces gross morphological and biochemical changes in the chloroplasts of infected cells. Viral RNA is synthesized in vesicles formed by invagination of the outer chloroplast bilayer. Virion assembly occurs at the neck of these vesicles and requires illumination. Data collected over the last three decades are consistent with the hypothesis that light-induced generation of a low pH drives TYMV assembly within the intermembrane space of chloroplasts. In a low-pH environment, poly(C) regions within the genomic RNA of TYMV may interact to form tertiary structures, and the recognition of these structures by TYMV coat protein initiates virion assembly.