The fish leech, Calliobdella nodulifera, transmitted a trypanosome while feeding on laboratory-reared cod ( Gadus morhua). The flagellate body length increased from 43 microm 17 days post-infection (d.p.i.) to 57 microm 113 d.p.i. Characteristic features are cell striation, a nearly central nucleus and a short (3.5 microm) distance from the kinetoplast to the posterior end (PK). Following growth, the trypanosomes became increasingly slender, with fewer striae and a shorter flagellum, while the PK was unaffected. The trypanosomes differ morphologically from Trypanosoma murmanensis transmitted by the leech Johanssonia arctica.