Abstract Schistosoma mansoni has been found to have a spectrum of purine nucleotides which is similar, but not identical to mammalian cells. The principal component of this system is ATP, which is present at a level of about 5·5 × 10 −9 moles/mg worm pairs. The ATP concentration falls if worms are incubated in a defined medium containing no purines. When worms are incubated in the presence of adenine or adenosine, ATP levels rise sharply and may be maintained at higher than normal levels. Schistosomes were found not to incorporate labelled glycine or glucose into purine nucleotide bases. In contrast, 14 C-8-adenine is taken up from the medium and can be detected in nucleotides, principally as ATP. In comparison with various mammalian cell systems, schistosomes show a much higher rate of uptake of free adenine. These findings suggest that schistosomes do not utilize the de novo pathway for the synthesis of nucleotides. It is postulated that the salvage mechanism is required to fulfil the worm's purine requirements. A new rational chemotherapeutic approach for schistosomiasis, based on interference with their specific nucleotide pathways, may thus become possible.