Among the most dangerous pests of potato crop is considered the nematode Ditylenchus destructor Thorne, 1945, obligatory parasite of tubers included in the list of quarantine species (e.g. EPPO), which causes damages of 30-80% during storage period in obtained in monoculture potato harvest. Nematode spread occurs with seed potatoes infested material, therefore according to international standard (E ЭKOOH S-1) the infestation with parasitic nematode species Meloidogyne chitwoodi, M. falax and Ditylenchus destructor is not permitted. Pathological processes that take place in the infected tubers are slow and prolonged. Paramonov and Briuşcova distinguished 5 phases of disease that are characteristic for Moldova (Melnik et al., 2014; 2015). Infested seed potatoes are most dangerous in the early stages of ditylenchosis (1; 2) as external symptoms, occurring on tubers in these phases, do not appear by careful selection, therefore such potatoes could be planted in the ground. The nematode D. destructor is included in the group of migratory endoparasites, with the possibility of leaving from the parasited plant tissue in the soil, regardless of developmental stages (larva or adult). The nematode feeds only on living cells and to mobility of population contribute to the widening of necrotic zones. In 2015-2016 there were conducted research to assess D. destructor nematode population structure in infested tissue potato tubers in the early stages of ditylenchosis (1 and 2) and the danger of using such tubers as seed material. In Phase 1 of ditylenchosis five varieties of potatoes – Agata, Romano, Rokko, Blue violet and Irga were obtained by inoculation with D. destructor (female + male). To determine the density of the nematodes laboratory analysis was performed after 35-40 days of incubation, during which the nematode not only penetrated the vegetal tissue of mesocarp but also reproduced. During the studies the presence of external symptoms of ditylenchosis in infested potatoes at this stage has not been established. Only by scalpel hollowing and removing the shell was discovered tiny yellowish specks of about 2 mm and the tissue was slightly macerated. The nematode presence in such tissue was determined by applying Baermann funnels. By microscopic analysis it was observed that the obtained suspension contain adult individuals of D.destructor, larvae J2 , J3, J4 and eggs. Dominant are larval forms, the most common being those just hatched from eggs (J2 ), more seldom – adult individuals. Density: larvae + mature individuals have increased considerably over the number of nematodes introduces in the process of inoculation, variable depending on the variety of potatoes. More sensitive to infestation, proved to be Blue violet and Irga (n = 450-970 individuals / gram of infested tissue) and less sensitive - Agata, Romano and Rokko (n = 120-235 individuals / gram). In phase 2 of ditylenchosis D. destructor density was evaluated in infested potatoes of variety Rokko, collected from deposits during storage. The infested portion of the tuber surface considerably widens: the diameter (d) - Length / Width = 15-20 x 16-22 mm and thickness of infested substrate = 4-5 mm. By removing the peel of potato on the pulp outskirts accumulation of D. destructor individuals is observed. In this phase, the nematode causes necrotic outbreaks. The conjunctive tissue deforms and infested portions detached from healthy tissue. Mesocarp in this section is distinguished from the healthy one by the color, which becomes yellowish or light brown. As in Phase 1, from such tissue a pure culture of D. destructor of different ages was extracted from, but their number is much higher. The most numerous are the forms of juvenile D. destructor (J2, J3, J4) with the dominance of J2 (n = 10.1 x 103 individuals / gram of infested tissue). The number of males (n = 3271.8 individuals / gram) is larger than that of females (n = 3046.8 individuals / gram). In suspension are also contained parasite eggs (n = 3375 units / gram). According to recalculations, D. destructor larvae (J2, J3, J4) constitute 50% of the total units / gram of infected tissue; the following are the eggs - 17.07%, then the males – 16.53% and the females – 15.42% (fig. 1). At a potato tuber infested in Phase 2 can be counted approximately 31.6 x 103 units of D. destructor. The number of larvae that feed intensively with healthy tissue of the potato mesocarp is high enough -10.1x103 and within 18-20 days they become mature stage. A single female can deposit around 200-250 eggs. In the Republic of Moldova D. destructor form 4-5 generations (Nesterov, 1970), and the intense reproduction of the parasite causes increase in the density of individuals. It is important to note that D. destructor can survive in soil up to 3-4 years on Solanaceae plant and grass remnants. To avoid cultivation seed potatoes infested with D. destructor in the early phases (1 and 2) of ditylenchosis it is necessary: to prohibit the use of potatoes produced on infested plots; to avoid potato cultivation in monoculture; to use methods of potato cultivation in conditions of mixed farming with the introduction of crop rotation. Aknowledgement: The work was performed within the project 15.817.02.12F financed by Academy of Sciences of Moldova.