Carrot (Daucus carota L.) roots treated with 2-chloroethylphosphonic acid or uronide elicitor accumulated the phytoalexin 6-methoxymellein. The extracts of these cells catalyzed methylation of 6-hydroxymellein to 6-methoxymellein with S-adenosyl-L-methionine as a methyl donor. Activity of the O-methyltransferase was not found in fresh carrot roots but increased when they were treated with either type of elicitor in parallel with the increase in 6-methoxymellein content. The enzyme did not methylate 6-methoxymellein, 3,4-dehydro-6-methoxymellein, and some coumarin derivatives, though 3,4-dehydro-6-hydroxymellein accepted methyl moiety at the same rate as in 6-hydroxymellein.