In many nearshore marine systems, microalgae can be chronically exposed to anthropogenic and biogenic phenolic and halophenolic compounds that accumulate in surficial sediments. Although biodegradation of some phenolic compounds has been demonstrated in freshwater algae, this capability has not been tested in marine species. We examined a ubiquitous marine diatom, Thalassiosira sp. HP9101, for its capacity to tolerate, and/or utilize phenol and benzoic acid. We also examined the aromatic ring cleavage reactions of this diatom and its capacity to dechlorinate chlorophenolic compounds. Axenic Thalassiosira sp. cultures were grown at 60 µE m-2 s-1 (12: 12 h light: dark cycle) in the presence or absence of phenol, benzoate, catechol or protocatechuate and their growth kinetics and physiological responses determined. Thalassiosira sp. was inhibited by catechol and protocatechuate. Growth in the presence of 1 mM phenol was observed only after an extended lag and was quite slow. Growth in the presence of 0.25 mM phenol occurred after a much shorter lag and phenol was taken up by these cultures at an estimated rate of 0.08 fmol phenol cell-1 h-1. No inhibition of Thalassiosira sp. by 1 mM benzoate was observed. Growth stimulation by added phenolic substrates was not demonstrated, although phenol-supplemented Thalassiosira sp. produced substantial levels of protocatechuate 3, 4-dioxygenase and protocatechuate 4, 5-dioxygenase, the ortho-and meta-pathway aromatic ring cleavage enzymes, respectively. Only protocatechuate 4, 5-dioxygenase activity was detected in Thalassiosira sp. grown with 1 mM benzoate. Phenol supplemented Thalassiosira sp. was also capable of dechlorinating monochlorophenols, 3, 5-dichlorophenol, and 2, 4, 6-trichlorophenol. These reactions were NADH-dependent and were not observed in control cultures grown without phenol.