Three exogenous phenol-degrading bacteria, Alcaligenes sp. R5, Acromobacter sp. El and Alcaligenes sp. E2, were each inoculated into separate laboratory units of activated sludge fed on peptone, and then phenol loading was started. The control unit (no inoculum) required 10 d for complete degradation of phenol. Inoculation with strain R5 did not shorten this start-up period, whereas inoculation with strains El or E2 shortened it to 2 or 3 d, respectively. Population dynamics of the three strains were analyzed by enzyme-linked immunosorbent assay (ELISA) by which a population with a density of more than 1 x 10(8) cells per mi (background density) could be detected. The R5 population density was never greater than the background, whereas the El population increased to 2 x 10(9) cells per mi (approximately 50% of the total cell count) during the startup period, and fell to below the background 35 d after the inoculation. The E2 population initially increased to 8 x 10(8) cells per mi, and subsequently survived at approximately 2 x 10(8) cells per mi throughout the remainder of 40-d experiment. These results suggest that the inocula, strains El and E2, capable of forming the predominant populations, caused successful bioaugmentation which shortened the start-up period. In pure-culture experiments performed to investigate the growth of the strains on phenol and peptone, it was found that strain El was superior in terms of growth on peptone at low concentrations to the other strains, and strain R5 was inferior. Such an advantage of strain El was not observed during growth on phenol. From these results, it is proposed that one of the important properties of microorganisms for successful bioaugmentation, i.e. shortening of the start-up period, Is potential for rapid growth on coexisting nutrients, such as peptone in this study.