The inhibitors of IAA-oxidase contained in dark-grown wheat seedlings are studied in this paper. Boiled water extract from dark-grown wheat seedlings showed an inhibiting effect on the IAA-oxidase activity when in higher concentration. It showed, however, a promoting effect when in lower concentration. Moreover, it showed a little interference with Salkowski's color reaction of IAA. The inhibitors were extracted from the fresh materials with methanol, n-butanol, iso-propanol, acetone, acetonitrile or ethylacetate with ease. But with benzene, n-hexane or petroleum ether, extraction was difficult. In order to identify the inhibitors, we have undertaken some experiments using paper chromatography and obtained the following results: these inhibitors showed positive color reactions with FeCl3 and diazo (alkaline) reagents, but negative with Al+++ and diazo (acid) reagents. As a result, it is suggested that these inhibitors are phenolics encluding flavonoids and catechins. From further studies using two-dimensional paper chromatography, it may be concluded that the IAA-oxidase inhibitors contained in dark-grown wheat seedlings are ferulic acid and other less active phenolcarboxylic acids.