Carrots (Daucus carota L.) are a ubiquitous host for arbuscular mycorrhizal fungi whereas sugar beets (Beta vulgaris L.) are a non-host. Root cultures were used to compare the constitutive phenolic compounds associated with the cell wall or present in the cytoplasm of the host and non-host. Phenolic acids were released from purified cell walls by alkaline hydrolysis and were separated and identified by HPLC, TLC and u.v. absorption spectra analyses. Two phenolic acids unique to carrot root cell walls were identified as p-hydroxybenzoic acid (p-HBA) and vanillic acid. Sugar beet root cell walls had ferulic acid as major constituent and contained several unique phenyl propanoids which were not identified. Caffeic acid was found only in the cytoplasm of carrot roots and was present in the conjugated form (chlorogenic acid). The sugar beet cytoplasm also contained several unidentified hydroxycinnamic acid-type phenolics which were not found in carrot roots.