Under field conditions, exposure of Aedes provocans to Plagiorchis noblei cercariae reduced the number of pupae produced to about one-third of control values. Larvae bearing more than 2 metacercariae rarely survived, and pupal mortality was almost 3 times higher among exposed individuals than controls. Adult females exposed to cercariae as pre-imagos experienced a 32% reduction in longevity. The life span of males was not affected. The potential of entomophilic digeneans as biological agents in the control of mosquitoes is discussed.