AbstractReproductive rates of three North American isolates ofBursaphelenchus xylophilusand a Finnish isolate ofB. mucronatuswere compared by culturing the nematodes at constant and at daily fluctuating temperatures in the laboratory and in the field.Botrytis cinereaandPinus sylvestriswood were used as nutrient sources. In general, theB. xylophilusisolates reproduced better than theB. mucronatusisolate at constant temperatures, whereas the situation was reversed at fluctuating temperatures.