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In this study, the effects of b-amino-butyric acid (BABA) on root-knot nematode(Meloidogyne javanica) infection of cucumber and accumulation of total phenolic compounds, hydrogen peroxide and activity of some enzymes related to plant defense mechanisms, i.e., guaiacol peroxidase (GPOX), polyphenol oxidase (PPO), catalase (CAT) in cucumber roots infected with nematode were investigated. Results of this study show that treating the cucumber seedlings with the above elicitor significantly reduces the nematode infection level (the nematode galls, number of egg masses per plant and number of eggs per individual egg mass) compared to control. Additionally, treatment of cucumber roots by BABA and BABA ? nematode, significantly increased peroxidase, polyphenol oxidase and catalase activities in root tissues, 1 day after nematode inoculation in comparison to nematode inoculated plants as control and sterile water-treated plants. Enzyme activities reached to a maximum level at 4, 4 and 3 days after nematode inoculation, respectively. Additionally, the amount of H 2 O 2 , a product of oxidative stress, was significantly increased in the BABA and BABA ? nematode treatments in comparison to control. Such increases have occurred in two phases and maximum levels of it were observed at 5 days after inoculation. Inoculation of cucumber plants by BABA also significantly increased accumulation of total phenol in comparison to control and maximum level of it was observed at 7 days after nematode inoculation. The results suggest that the inhibitory effect of BABA on the root-knot nematode (M. javanica) may be related to its ability to enhance defense responses in the cucumber roots.