Recently we have showed that the expression of the mammalian CYP11A1 cDNA in plants confers their resistance to abiotic and biotic stresses. To determine the role of heterologous expression of cytochrome P450scc cDNA in resistance to ROS (radical oxygen species) dependent abiotic stresses, the structural changes of mitochondria and peroxisomes were studied under 150 mM NaCl-induced 14-day salinity treatment on juvenile tobacco plants in in vitro culture. Ultrastructural analysis of mesophyll cells of transgenic tobacco leaves constitutively expressing CYP11A1 cDNA was performed. Under NaCl stress, a change in shape from rounded to elon-gated, reduced section area, formation of branched mitochondria, as well as the emergence of triangular and rhomboid cristae, densification of a mitochondrial matrix, increase in density of contrasting membranes and their thickness were observed in non-transgenic plants. Transgenic plants without stress applied had mitochondria with rounded and elongated shape, twice as small as in non-transgenic plants, with a dense matrix and sinuous cristae. Surprisingly, the effect of NaCl led to increase in size of mitochondria by 1.5 times, decomposition of matrix and the emergence in organelles of light zones presumably containing mitochondrial DNA strands. Thus, the structural organisation of transgenic plant mitochondria under salinity treatment was comparable to that of non-transgenic plants under native conditions. It was also noted that the transgenic plant peroxisomes differed in non-transgenic tobacco both in normal condition and under the action of NaCl. The observed differences in ultrastructural organisation of mitochondria not only support our earlier notion about successful incorporation of the mature P450scc into this organelle, but for the first time demonstrate that the mammalian CYP11A1 signal peptide sequence could be efficiently used in the formation of targeted mitochondria protection of plants from salinity-induced damage.