Plasmodium berghei ookinete formation in vitro and within the midgut of susceptible Anopheles atroparvus were compared. No significant morphological differences were seen, except that in vitro development was more synchronized and less degenerating forms occurred. In vitro ookinete yields were 4-31 times higher and less variable than those in vivo. Mosquitoes of a susceptible and of a refractory line of A. atroparvus were simultaneously fed on the same host or via a membrane with the same suspension of in vitro-formed mature ookinetes. Up to 100% of mosquitoes of the susceptible line produced oocysts, mostly in high numbers, whereas infection rates and numbers of oocysts produced in mosquitoes of the refractory line were lower and much more so after host feeding than after membrane feeding of mature ookinetes, indicating that refractoriness does not depend on a single process of inhibition.