Light is an important factor of the plant's environment. The aim of research was to confirm the hypothesis on the influence of different irradiation levels on the fluctuating asymmetry (FA) value of bilateral traits of plants cultivated in the indoor plant lighting. The object of research was the plants of cucumber (Cucumis sativus L) as one of the main glasshouse crops. Young 14-day-old cucumber plants in the first true leaf phase were studied. Different irradiation levels (15.0, 22.5 and 30 W m-2) were maintained by fluorescent lamps. An essential asymmetry of bilateral structures in plants grown under different irradiation levels was observed. It was found that statistically significant lower values of FA, that is greater stability of plant development, correspond to increased plant performance. When the irradiation level was switched from 15 to 30 W m-2(by 100%), the FA index for different bilateral structures varied by dif ferent amount: in terms of cotyledons mass it decreased from 0.046 to 0.032 relative units (by 30.2%), in terms of chlorophyll content in cotyledons it increased from 0.038 to 0.073 relative units (by 88.6%). In some bilateral structures the FA index varia tion was rather small: in terms of the cotyledon area it was zero, in terms of the cotyledon thickness – by 1.8%. I t was experimentally prove through the example of cucumber plants that FA index could be used as an indicator of plant developmental stability, characterizing the deviations of the growing environment parameters from the normal state in the indoor cultivating.