Factors influencing the dissolved oxygen balance in an Australian stream are examined. A mathematical description of the balance is developed supplementing the formulations of Dobbins (1964) and O'Connor (1967). A computer model (OXSTRAL) of the balance has been constructed and shown to be valuable for predicting levels. Application is illustrated using data form the alpine Perisher Creek. The need to conduct a sensitivity analysis of the model to assess the significance of natural variations and/or errors in the data input was evident. The principles underlying the sensitivty analysis are discussed and the value of the analysis as a method of ascertaining data requirements is demonstrated.