An acoustic actograph was used to monitor for 17 days after infection the spontaneous flight activity of the mosquito Anopheles stephensi parasitized with the rodent malaria Plasmodium yoelii. Activity fell to approximately two-thirds of control levels at about day 10 post-infection--when oocysts were reaching maximum size and starting to rupture (mean number of oocysts = 92)--and thereafter remained at this reduced level. The circadian activity pattern was not affected by the parasitism.