Ctenocephalides felis felis larvae were infected with Dipylidium caninum at a range of temperatures from 20 degrees - 35 degrees C at 3 mm Hg saturation deficit (SD) and 30 degrees C at 8 mm Hg SD. Hosts were subsequently dissected at 6, 9 and 12 days after infection. Four replicate experiments were performed and results of development, and host reactions analysed by the Genstat computer programme. These were found to depend on the temperature and saturation deficit of the environment. Unlike previous findings, parasite development and host reaction were found to be independent of host development. Host reaction was more marked and prolonged at 20 degrees - 25 degrees C than at higher temperatures. No perceptible growth of the parasite occurred at 20 degrees C. The development patterns of growth at the higher temperatures were similar but shifted in time so that faster growth occurred at higher temperatures. Rate of growth was fastest at 35 degrees C, despite the fact that this temperature was unfavourable to the hosts, all of which died at the time of pupation.