A toxicological assessment of water quality conditions (30 physical-chemical parameters and a screening of organic pollutants by reverse-phase HPLC) was performed in Algeciras Bay, Cadiz, Spain in relation to problems in six floating ponds for gilthead (Sparus auratus) growth. The study was completed with a short-term Artemia toxicity test, and the measurement of heavy metal concentrations in molluscs from different sites of the Bay. Cadmium, lead, and cyanide at sublethal toxic concentrations were found in most of the analyzed samples, and very high concentrations of ammonia, phenols, nitrites, cyanide, and sulphide were found in some sampling sites. In addition, seven water samples showed toxicity on Artemia. The problems in the floating ponds (low growth rates, high mortality, and infectious diseases) can be related, in part, to the very bad water quality conditions. © 1991 Springer-Verlag New York Inc.