20-hydroxyecdysone (20E)is the major phytoecdysteroid of about 6% of plants. Its role in plant physiology has not been fully elucidated. In this work we studied the effects of 20E application on some morphological and biochemical parameters of tomato, Lycopersicum esculentum, seed during germination and seedling development (5 d). We compared the 20E effects with the action of phytohormones: gibberellic acid (GA 3 ), naftalen acetic acid (NAA), benzyl amino purine (BAP). NAA treatment resulted in marked reduction in shoot length. GA 3 treatment promoted maximal shoot elongation. BAP affected negatively shoot length only at late stages, while 20E application stimulated shoot elongation at early stages and reduced it on the fifth day; NAA inhibited root elongation all along the test period. GA 3 treatment had no effect on root length, whereas BAP showed strong inhibition on root elongation. On the other hand, 20E showed a weak inhibition of root elongation on the fifth day. As compared to control and to other treatments, NAA and 20E provoked a drastic decrease in protein contents during seedling growth, whereas a high increase was observed under BAP treatment. Electrophoresis revealed that protein bands were not degraded and mobilized after NAA treatment while in control or after 20E and others phytohormones applications, protein patters displayed weak band intensities and some of them were not detected. NAA, GA 3 and BAP provoked a decrease in proline content during seedlings, while the effect of 20E on proline levels varied during germination and plantlet development. This work showed that 20E like phytohormones fulfil some bioactive actions during germination and seedlings growth in tomato.