We present an easy and sensitive method for measuring thiamine and its phosphate esters in small biological samples of microalgae (Amphidinium carterae Hulburt and Nitzschia microcephala Gran). The method consists of extraction of thiamine and its derivatives in acid solution, followed by liquid chromatography with fluorescence detection. The detection limit is as low as 15 fmol of thiamine. For comparison to microalgae, the method has been applied to evaluate thiamine levels in the crustacean Artemia salina Leach and is suitable for nutritional studies of the food web of the Baltic salmon, which suffers from thiamine deficiency. This method of HPLC analysis can be readily utilized to follow uptake and interconversion of thiamine and its phosphate esters in many micro- and macro-algae. (C) 2002 Elsevier Science (USA).