The effect of low water temperatures on the development and viability of larval and adult Ang~lillicolac rassus (Nematoda) in the final host Anguilla ang~lillaw as studied. European eels were experimentally infected with A. crassirs and then maintained for 4 mo at 4,9, 10 and 19°C. Larval development showed a temperature-dependent pattern and was significantly retarded at low temperatures. Third-stage larvae survived a 4 mo period at 4'C without belng affected, although they were not able to invade the swimbladder wall at this temperature. In contrast, adult worms were severely harmed during a 4 mo period at 4'C, as reflected by increased mortality and decreased growth and reproductivity compared to the worms maintained for Lhe same period at 18°C. Starvation of the eels for 4 mo at 19°C dld not affect the development and growth of the nematode The experimentally obtained results support the hypothesis that the spread of A. crasslrs in boreal regions, e.g. Northern Europe, IS restricted by the natural ambient temperature regimes.