Arthrobacter strain ATCC 33790, a pentachlorophenol (PCP)-metabolizer isolated by the author, has been recovered after 10 years of storage. The freeze-dried preparation grown on half-strength Trypticase Soy Broth adapated to utilize PCP within 1 week. Cultures grown on PCP-nutrient agar were found to utilize PCP in mineral salts medium within 2-3 days. The culture was prepared for continuous growth at pH 6.5 by successive feeding of 100-110 mg solid aliquots of PCP to a 1-1 culture initially grown at pH 7.4. Continuous culture growth at pH 6.5 was possible on a mineral salts feed containing 1800 ppm PCP. Continuous cultures grown at pH 6.7 on mineral salts feeds containing 500 and 340 mg PCP/1 were especially efficient in removing PCP. Less than 4 mg PCP/1 were detected in the effluent at dilution rates near washout. In batch culture studies at pH 6.5 the PCP utilization kinetics were found to be similar at low PCP concentration to those at pH 7.4 for the approximately same inoculum size. Utilization of 35 mg PCP/1 was very slow at pH 6.0. Growth rates at pH 6.5 at controlled PCP concentration ranges of 5-35 and 75-115 mg/l were 0.09 h(-1) and 0.05 h(-1), respectively. The ability of strain ATCC 33790 to utilize PCP in mineral salts media containing naphthalene, methylnaphthalenes, and cresols was examined. Naphthalene, 1-, and 2-methyl-naphthalenes at their solubility limit, and o- and m-cresols at 900-1000 mg/l prevented utilization of 80-90 mg PCP/1. PCP was rapidly removed from both commercial sand at 30 degrees C and from clay soil at room temperature. Estimated inoculum sizes of 6.6x10(6) 6.6 x 10(4), and 656 cells/g were found to be effective in removing approximately half the starting amount of PCP from sand in 3, 19, and 42 h, respectively. Nearly complete disappearance of extractable PCP was observed after 1 day in clay soil inoculated with 6X10(6) cells/g.