Technical Abstract: The effect of temperature on the production, survival and infectivity of zoospores of an Argentinean isolate of Leptolegnia chapmanii was determined under laboratory conditions. Production of zoospores of L. chapmanii in vitro and in vivo upon first and fourth instars larvae of the mosquito Aedes aegypti was studied at three different temperatures. Zoospores from infected larvae were infective to mosquito larvae for 51, 12, and 5 consecutive days when maintained at 25, 35, and 10 ºC, respectively. Maximum zoospore production in infected fourth-instar larvae was 9.6 ± 1.4 x 104 zoosp/larva at 48 h at 25 ºC. The average number of zoospores produced by individual fourth-instar Ae. aegypti larvae infected with L. chapmanii was 3.57±0.46 x 105 zoospores during 6 consecutive days at 25 ºC. Zoospore production in vitro was also affected by temperature with a maximum of zoospores (n=47,666/ml) produced at 25 ºC. When zoospores produced in vitro were used as inoculum against Ae. aegypti larvae at 25 ºC, larval mortality was recorded for 5 consecutive weeks. The encystment process for zoospores took 17¿20 minutes; the germination of cysts (excystment) occurred 5 minutes after exposure in water to mosquito larvae. The minimal time of contact between zoospores and mosquito larvae to develop infection was two minutes. Infection took place by zoospore attachment onto and then penetration through the larval cuticle or by ingestion of cysts as was confirmed by histological studies. Temperature directly affected infectivity and production of zoospores in vivo and in vitro although L. chapmanii zoospores tolerate a wide range of temperatures.