A cDNA clone encoding rice tryptophan synthase β (TSB) was isolated and its transcript level was examined in TR-No. 73, a derivative of a previously isolated rice mutant (TR-1) that is resistant to 5-methyltryptophan. The cDNA sequence of the rice gene for TSB was very similar to that of genes for TSB from other plants. Northern blotting analysis revealed that the steady-state level of TSB mRNA in the 5MT-resistant mutant TR-No. 73 was 1.3 times higher than the level of TSB mRNA in control rice plants under standard conditions. The level of TSB mRNA in control rice plants decreased after treatment of plants with 5MT. Similarly, the level of TSB mRNA in TR-No. 73 initially decreased, although less so than in control rice. However, after 12 h of treatment with 5MT, the level of the transcript in TR-No. 73 returned to the level under standard conditions. The activity of tryptophan synthase (TS) in seedlings of TR-No. 73 was about 2.3 times higher than that in seedlings of control rice under standard conditions.