1. The cellular nutrient contents of microalgae, when growing at or approaching maximum rates, approximate the Redfield C:N:P (molar) ratio of 106:16:1. Deviations from this optimal ratio can be used to infer nutrient limitation of microalgal growth. However, this ratio may not be applicable to macroalgae, which are distinguished from microalgae by forming a thallus that is a discrete structure visible to the naked eye. The utility of the Redfield ratio to infer nutrient limitation of the growth of macroalgae was tested for Spirogyra fluviatilis in a field experiment conducted in tropical Australia. 2. The optimal cellular C:N:P ratio for S. fluvialitis was estimated by means of in situ nutrient addition. This was compared with S. fluvialitis cellular ratios determined from eight sites with a wide range of soluble N concentrations (<1-90 μg L-1), a smaller range of soluble P concentrations (5-12 μg L-1), and soluble molar N:P ratios of 0.11- 27. 3. Spirogyra fluviatilis had an optimal molar C:N:P ratio of 1800:87:1 which differs substantially from the Redfield ratio, and suggests that the latter ratio is not applicable to this macroalga. Concentrations of N and P in the river deviated from the optimal N:P ratio of 87:1, inferring nutrient limitation of growth. 4. C:P and C:N ratios of S. fluviatilis varied in accordance with general stoichiometric relationships for autotrophs under nutrient limitation of growth. Ratios of C:P and C:N increased, respectively, with increased severity of P- and N-limitation. Additionally, C:P ratios increased with increased N:P ratios, whilst the C:N ratio increased with decreased N:P ratios. The C:N molar ratio however was an insensitive indicator of nutrient depletion compared with the C:P ratio. Under N-limitation of growth, luxury amounts of P were stored by S. fluviatilis. 5. In aquatic environments where macroalgae are sufficiently abundant to be sampled, their cellular carbon, nitrogen and phosphorus stoichiometry can be used to infer nutrient limitation of growth when their optimal C:N:P ratio is known.