The development, survival and infectivity of larvae of Nippostrongylus brasiliensis at 10 and 21 degrees C have been studied. In cultures developed for 1 week at 21 degrees C but stored at either 10 or 21 degrees C the mean recovery of larvae in 1.5 h Baermann sediment (46% and 42% of total eggs, respectively) and their infectivity (66% and 70% day 8 p.i. recovery, respectively) at 4 weeks of cultivation were similar. However, at 20 weeks of cultivation only a few larvae survived storage at 21 degrees C (1.4%) and the survivors showed very low infectivity (4.1%), whereas larvae stored at 10 degrees C retained most of their survival and infectivity rates (41% and 44%, respectively). In cultures developed and stored at 10 degrees C a lower mean proportion (17%) of the eggs was recovered as larvae at 4 weeks of cultivation. The mean infectivity rate of these larvae increased from 4 to 12 weeks of cultivation (18% and 42%, respectively) but was reduced at 20 weeks of cultivation (27%). An increased proportion of adult female worms was recovered from rats infected with larvae from cultures showing poor development or survival and the fecundity of the adult females decreased in all groups as the storage time of the cultures was increased. Eggs from low fecundity adult worms showed normal development of larvae. The relation between survival, temperature in the habitat and the geographical distribution of the larvae of N. brasiliensis is discussed.