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Relevance. The aim of the study was a comprehensive evaluation of hypocotyl length in breeding lines of cucumber as a morphological trait affecting seedling resistance to lodging, as well as an assessment of the stability of this trait depending on growing dates. Materials and Methods. The studies were carried out in 2019-2021 and 2025 in the seedling compartment of a winter greenhouse at the FSBSI Federal Scientific Vegetable Center (Moscow Region, Russia) using breeding lines of cucumber ( Cucumis sativus L.). Each year, about 30 lines were evaluated, and hypocotyl length was recorded for the most contrasting genotypes. Sowing was performed in March and May; seedlings were grown in 0,7 L pots without supplementary lighting. Hypocotyl length was measured at the cotyledon and first true leaf stages on 10-16 plants per accession. Data were analyzed using one-way ANOVA with Fisher’s LSD and Duncan’s multiple range test at p≤0.05. Results. Analysis of hypocotyl length, internode length, and plant height showed that these traits were predominantly genotype-determined (72% contribution). The effects of year and the “line × year” interaction were statistically significant but less pronounced. Comparable light conditions in 2021 and 2025 allowed year-to-year differences in the studied traits to be interpreted mainly as a result of genetic variability. According to Duncan’s test, the lines were grouped into three classes based on hypocotyl length: short (3,8-5,4 cm), medium (6,1-7,6 cm), and long (6,2-8,8 cm). Lines with a short hypocotyl generally showed a higher coefficient of variation than those with a longer hypocotyl, indicating the feasibility of selection for this trait. Of particular breeding interest was line Ur., which combined a short hypocotyl with a lower coefficient of variation compared with other short-hypocotyl lines. Conclusion. Correlation analysis revealed a strong relationship between hypocotyl length and internode length (r=0,76), reflecting their coordinated development, while the effect of hypocotyl length on plant height was weak (r=0,15). This allows breeding for seedling lodging resistance in lines differing in plant height. Lines with a short hypocotyl conferring increased lodging resistance were identified and can be used in breeding medium-height (Ur., Gar.) and tall (Mer., Mon.) parthenocarpic cucumber hybrids for spring greenhouses and temporary shelters.