Concentrations of phosphate in natural waters are often below the detection limits of conventional nutrient autoanalyzers, by either gas-segmented continuous-flow analysis orflow injection analysis. A liquid waveguide capillary flow cell has been used to extend the sensitivity of a conventional autoanalyzer for the automated analysis of nanomolar concentrations of phosphate in natural waters. Total reflection of light can be achieved within the liquid core of the flow cell because the refractive index of a cell wall coated with Teflon 1600 is lower than that of water. This property allows the manufacturers to construct long liquid waveguide capillary flow cells in a helical, rather than a linear shape, with compact dimensions. A small sample volume is required because the internal volume of a 2-m long capillary flow cell is only approximately 0.5 cm3. Adaptation of this long flow cell to autoanalyzers significantly enhances the sensitivity of automated colorimetric analysis of phosphate with a molybdenum blue method, allowing for the accurate and precise determination of nanomolar concentrations of phosphate in natural waters. The advantages of this technique are a low detection limit (0.5 nM), a small sample volume (2 mL), high precision (2% at 10 nM levels), and automation for the rapid analysis of a large number of samples.