Phenols are one of the most common groups of organic environmental pollutants. In this study, the catabolic versatility of the eukaryotic alga Ochromonas danica (CCAP 933/2B) to degrade mixtures of, rather than individual, phenols was examined. O. danica, after growth on phenol, was able to metabolise 2,5-, 2,6- and 3,5-xylenols only in the presence of phenol in the incubation medium. Once the phenol had been exhausted (2-3 h) from a phenolic mixture in the incubation media, there was a significant reduction in the rates of xylenol isomer removal and none of these xylenols (250 mu M) was completely utilised in 6 h. 3,4-Xylenol, however, was completely removed from the growth medium in 3 h, even though all the phenol had disappeared in 2 h.