Summary Growing photoheterotrophic cultures of Euglena gracilis Z show a time-dependent loss of chlorophyll per cell that results in the production of bleached cultures of the alga on prolonged incubation at 33°C. In terms of chlorophyll accumulation, the bleaching process involves an initial rise in chlorophyll accumulation per cell, peaking at about 15 h, followed by a second phase in which chlorophyll levels per cell actually decline. The primary objective of this work was to establish whether loss of chloroplast function at 33°C is associated with a loss of translational activity on chloroplast ribosomes or a loss in the synthesis of chloroplast proteins of nucleocytoplasmic origin, or both. To this effect, pulse-labeling experiments were carried out in vivo on cultures of the alga in the second phase of the temperature response. The results show that synthesis and import of proteins from the nucleocytoplasmic compartment remain high in bleaching plastids during the first 41 h of the temperature treatment. By contrast, protein synthesis inside plastids is gradually lost at the elevated temperature, and only low levels can be detected in 41-h bleached cultures. Consequently, the great majority of the proteins accumulating in 41-h bleached plastids are imported from the nucleocytoplasmic compartment. The loss of translational activity inside the bleaching plastid could explain why Euglena is unable to maintain photosynthetically competent plastids at the elevated temperature.