Trace amount of cobalt in water samples was preconcentrated continuously with an organic precipitant and determined by flame atomic absorption spectrometry. The flow injection technique was used to preconcentrate cobalt by on-line direct precipitation with 1-nitroso-2-naphthol. The precipitation was dissolved with methyl isobutyl ketone (MlBK) and was sent to the flame. The optimum conditions for cobalt determination were determined and used to analyze Co samples. For 1.0 mL of sample, the enrichment factor was 13 and the sample throughput was about lO per hour for 0.5 ppm Co solution. The enrichment factor was increased to 68 fold for 10.0 mL. A semi-reference biologicaI sample was prepared and analyzed. The result was in good agreement with the expected value with RSD of 4%.