In soil, high variability of U bioavailability results in large range of apparent U toxic levels for plants. U toxicity on hairy roots of carrot was studied in nutrient gel with a standardized in vitro device. After exposure to 2.5 and 20mgUL−1 for 34 days, U concentration ranged between 4 and 563mgUkg−1 fresh weight which was in good accordance with U accumulation by roots of plant from contaminated soils. Threshold ofUtoxicity for root length decreased with time and a transient hormesis occurred for exposure to 2.5 and 5mgUL−1. After 34 days and with root length as endpoint, significant toxicity appeared at a gel contamination level above 7.5mgUL−1 corresponding to a maximum U concentration in the liquid phase of 0.8mgL−1. The low values observed in this study could result from high sensitivity of carrot to U, high bioavailability of U in gel or absence of interferences with microorganisms. This in vitro device appeared adapted to study toxicity of U to plant roots in optimal conditions of both exposure and observations and is recommended to examine further physiological processes and the influence of microorganism interactions.