In a survey of the bacteria in swimming pools treated with either chlorine or Baquacil in private gardens in Melbourne, the water was frequently found to be at the incorrect pH and to contain biocides at suboptimal concentrations. The general bacterial flora count was commonly greater than 200 per mL; only 14% of chlorine-treated pools and 27% of Baquacil-treated pools consistently gave counts of less than 200 per mL. Coliforms were detected in 66% of chlorine-treated pools and in 22% of Baquacil-treated pools. Escherichia coli was detected in 32% of chlorine-treated pools and 8% of Baquacil-treated pools. Staphylococcus aureus was detected in 36% of chlorine-treated pools and in 8% of Baquacil-treated pools. Pseudomonas aeruginosa was detected in 7% of chlorine-treated pools, but not at all in Baquacil-treated pools. When the biocides were maintained at the correct concentration, the indicator organisms were well controlled by both biocides. This survey indicates that owners of pools need to be made aware that their pools can harbour potentially pathogenic bacteria unless biocides are constantly maintained at the correct concentration. Baquacil generally remains above the minimum recommended concentration for approximately 14 days between routine additions, whereas chlorine can dissipate from the pool within hours of addition on hot sunny days. This probably contributed to the superior over-all performance of Baquacil in this survey.