Because phenols are one of the most common groups of organic pollutants in the aquatic environment, heterotrophic growth-linked biodegradation of phenol and its methylated homologues by the eukaryotic alga Ochromonas danica (CCAP 933/2B) was investigated. The alga grew heterotrophically on phenol and mixtures of phenol with o- or p-cresols, or with 2,5-, 2,6-, 3,4- or 3,5-xylenols as the sole sources of carbon in the dark at 25 degrees C. Commensurate with growth, the alga removed phenol, both cresol isomers and 2,5- and 3,4-xylenols from the growth media over the incubation periods. In every case, phenol was removed preferentially to the methylated cosubstrates, but the rates of removal for phenol were slower than in incubations where phenol was the sole carbon source.