Twelve week old tobacco, N. tabacum L. cv. Little Turkish, plants were infected with tobacco mosaic virus (TMV) and ultrastructural effects in leaf and root tissue were examined with respect to cellular organelles. The infected mesophyll tissue had swollen chloroplasts with bulging, electron dense opaque bodies and distorted lamellae. Mitochondria were enlarged with swollen cristae. The plasma membranes were electron dense and showed discontinuities. TMV particles invaded the cytoplasm of most TMV-infected cells with evidence of heightened ribosomal activity. Doughnut-shaped nucleoli were present in infected cell nuclei with the chromatin becoming more granular in appearance. Fine structural examples of TMV-infected leaf and root cells are described.