Tetracycline antibiotics, both tetracycline (TC) itself and the chemically-similar oxytetracycline (OTC), are widely used in U.S. fish culture, but there have been few studies on the environment fate of waste antibotics that are released in the farm effluent and the effect of these residues on indigenous microbial communities.  One component of this study examined the fate and effects of TC used as a bath treatment at a California sturgeon farm, with medicated effluent discharged to a small stream. While TC residues were chemically detectable both in the effluent and in sediments of the receiving waters, these resides were not of sufficiently high concentration to elicit a measurable response in sedimentary microbes. The total microbial abundance was unaffected, there was no increase in the proportion of the community which was antibiotic resistant, and samples of the sediment containing TC residues failed to show any affect on the growth of a bacterial strain known to be TC-sensitive. In aquatic microcosms spiked with TC, concentrations of nearly 100 yg/g in the sediments were insufficient to elicit any microbial response, indicating that the environment concentrations observed at the farm of <5 yg/g were well below microbially-active levels by the measures of effect used herein. This study provided clear evidence that the microbial effects of aquacultural drug residues are highly dependent upon physical/chemical factors of the sediment affecting drug bioavailability and/or the composition of the native microflora and their inherent sensitivity to antibiotics. When six sediments were spiked with OTC, two of them showed antimicrobial potency at environmentally realistic OTC concentrations (<20yg/g). In the other four sediments bacterial numbers and growth was affected only at extraordinarily high OTC concentrations of 200 to 2,000 yg/g or more. Thus, concentration of OTC typically reported surrounding aquaculture facilities may have affects on bacterial densities in some cases, but most often will not. Effects, if any, may be measurable only by use of endpoints more sensitive than total bacterial density. It is not clear what factors determine whether a given level of antibiotics in sediment will have any antimicrobial effect. There was no apparent influence of salinity, although the role of sediment organic carbon content or some correlate was suggestive.