Electron microscopy of Russet Burbank potato (Solanum tuberosum L.) leaf midveins infected with the potato leafroll virus (PLRV) revealed the pathological effects induced by the virus in the phloem. Virus particles were observed in mature sieve elements, companion cells, and plasmodesmata connecting the two kinds of cells. An early indication of virally induced cellular disturbance in companion cells was dilation of mitochondrial cristae followed closely by the appearance of vesicles in the parietal cytoplasm. One type of vesicle contained fibrils suggestive of nucleic acids and the second type carried an electron opaque material. Both types of vesicles were surrounded by two membranes. Rough endoplasmic reticulum (ER) formed the outer membrane. Both vesicle types fused with the nuclear envelope and were taken up into the nucleus. PLRV particles appeared mainly in the cytoplasm, where they were either scattered or localized along the membranes of mitochondria, chloroplasts, and vacuoles. In some cells, groups of particles were enclosed in the vacuole. The relation of virus particles to the nucleus remains problematic: there was no evidence that uptake of vesicles into the nucleus resulted in the formation of particles in this organelle. Cells in which particles formed became degenerated. Mitochondria swelled and cristae separated from the envelope and clumped. Most organelles including the nucleus, ribosomes, rough ER, and chloroplasts broke down leaving a fibrillar material interspersed with virus particles.