It is now well established that natural phytoplankton populations may liberate a substantial proportion of the carbon fixed in photosynthesis directly into the external medium in the form of dissolved organic matter. Glycollate may be an important component of this extracellular organic fraction, both in culture (Hellebust, 1965) and in the natural environment (Watt, 1966). Nevertheless, it has been difficult to investigate the role of glycollate in the environment because of the lack of a convenient and reliable method for determining its concentration in natural waters. Shah & Fogg (1973) and Shah & Wright (1974), however, have developed a procedure for glycollate analysis in sea water, based on adsorption on to alumina and subsequent elution for determination by the Calkins colorimetric technique, which is both sensitive and not too laborious for routine analyses. Fogg (1966) suggested that glycollate excreted by phytoplankton acts as an extracellular reserve which may be utilized for survival under conditions unfavourable for photosynthesis. Wright & Hobbie (1965) showed that some fresh water bacteria take up glycollate at rates of the same order of magnitude as for acetate and glucose. Glycollate would therefore appear to be of importance in the energy flow between different trophic levels. In order to evaluate these hypotheses it seems important to determine the concentrations and turnover rates of glycollate occurring in natural waters.