Photosynthate translocation in single leaflets of healthy and curly top virus-infected tomatoes was investigated using (14)C as a marker. The amount of radioactivity found in plant parts not exposed to (14)CO(2) was substantially lower in diseased than in healthy plants. The time lag for the appearance of (14)C in the petiole was considerably longer in the infected plants than in the healthy. The kinetics of disappearance of (14)C from the lamina during the 24-hour period following labeling showed a strong retention of recent assimilates within the diseased leaf, not accompanied by increased immobilization into insoluble forms. Sucrose was the predominant compound participating in photosynthate transport in both healthy and diseased leaves. The amount of (14)CO(2) fixed was approximately 40% lower in curly top virus-infected leaves than in healthy leaves.