Pharmaceutical compounds (e.g. prescription and non-prescription drugs, steroids, hormones) are often found in ground and surface waters of the United States and other countries. These compounds may find their way into ground and surface waters through either wastewater treatment plants or septic systems, which are not designed to remove these compounds from the effluent. This includes antibiotics, many of which pass through the human body and are excreted largely unaltered. The development of antibiotic resistance is a major worldwide concern. It is thought to be most easily developed when a bacterial population is exposed to low concentrations of an antibiotic. This situation is likely to occur when antibiotics are used domestically and then excreted and diluted into the wastewater effluent leaving the household. The development of tetracycline resistance in Escherichia coli and fecal streptococci (Enterococcus faecalis and Enterococcus faecium) was examined, and the effects of tetracycline on microbial biomass and community structure in lysimeters representing the leachfield of a domestic septic system under both aerated (AIR) and conventional (LEACH) conditions were analyzed. A membrane filtration method and antibiotic disk diffusion were used to determine tetracycline resistance, and phospholipid fatty acid (PLFA) analysis was used to determine community structure and microbial biomass. There was no indication that antibiotic resistance in fecal indicator bacteria increased in the AIR or LEACH soils or drainage water over the course of the experiment due to the addition of tetracycline to the soil lysimeters. PLFA data showed no significant differences in prokaryotic or eukaryotic concentrations of PLFAs in either the AIR or LEACH soils before and after the addition of tetracycline.