Trypanosomes were studied from the blood of 6 species of small British fish caught in the River Lee. Morphologically the trypanosomes from the blood of Nemacheilus barbatulus L., Phoxinus phoxinus L., Cottus gobio L., Gobio gobio L., Gasterosteus aculeatus and Pungitus pungitus L. were indistinguishable. Cross-transmission experiments using syringe passage of culture forms and also the leech vector Hemiclepsis marginata showed that the trypanosomes were not host specific. The isoenzyme patterns of culture forms from N. barbatulus and P. phoxinus were identical for 11 enzymes studied. The trypanosomes from the 6 species of fish previously classified as separate species are, on the basis of these results, regarded as a single species, Trypanosoma cobitis (Mitrophanow, 1883).