A 17beta-estradiol-utilizing bacterium, Sphingomonas strain KC8, was characterized in terms of its utilization kinetics toward 17beta-estradiol, estrone, and testosterone. The maximum specific substrate utilization rates (q(m)) are 0.37, 0.50, and 0.17 mg-substrate/mg-protein/day for 17beta-estradiol, estrone, and testosterone, respectively. The half-velocity constants (K(s)) are 1.9 mg/L for 17beta-estradiol, 2.7 mg/L for estrone, and 2.4 mg/L for testosterone. Strain KC8 can grow on testosterone, glucose, sodium succinate, and sodium acetate, but not on phenol. Also, strain KC8 cannot degrade two common wastewater micropollutants, bisphenol A (a plasticizer) and triclosan (an antimicrobial agent). Unlike Novosphingobium sp. ARI-1 (a known estrogen-degrader) that would lose its degradation ability toward estrone after growing on a nutrient-rich estrogen-free medium for 7 days, strain KC8 was still able to degrade both 17beta-estradiol and estrone after growing on the same medium for 15 days. Strains KC8 and ARI-1 were molecularly detected in activated sludge of municipal wastewater treatment plants (WWTPs) operating under solid retention times of 2-30 days. The concentrations of strain KC8 were 2-3 orders higher than those of strain ARI-1 in the WWTPs, suggesting that strain KC8 is ubiquitous in WWTPs and might play an important role in estrogen removal.