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Antibiotics can be present in low concentrations in domestic wastewater, but little is known about their effect on bacteria in onsite wastewater treatment systems. Mesocosms, consisting of soil-filled lysimeters representing the leachfield of a septic system under aerated (AIR) and unaerated (LEACH) conditions, were used to study the effects of tetracycline addition (5 mg L(-1)) to septic tank effluent on tetracycline resistance in the fecal indicator bacteria Escherichia coli and fecal streptococci, and on their removal. The mesocosms were dosed with antibiotic for 10 days, and effects monitored for 52 days. The fraction of resistant bacteria in mesocosm drainage water relative to that in septic tank effluent, GammaRes, for E. coli ranged from 0 to 0.66 in the AIR treatment and from 0 to 3.32 in the LEACH treatment. For fecal streptococci, GammaRes ranged from 0 to 0.41 and from 0.63 to 1.06 in the AIR and LEACH treatments, respectively. No significant differences in antibiotic resistance of fecal indicator bacteria were observed among sampling dates in soil or water from either treatment. Tetracycline had no significant effect on removal of fecal indicator bacteria, which ranged from 99.9 to 100% for E. coli and from 95.9 to 100% for fecal streptococci. Our results suggest that short-term addition of tetracycline at environmentally-relevant concentrations is likely to have minimal consequences on pathogen removal from wastewater and development of antibiotic resistance among pathogenic bacteria in leachfield soil.