Hetsko, M. L., McCaffery, J. M., Svärd, S. G., Meng, T.-C., Que, X., and Gillin, F. D. 1998. Cellular and transcriptional changes during excystation ofGiardia lamblia in vitro. Experimental Parasitology88, 172–183. Excystation ofGiardia lambliaentails differentiation of dormant cysts into parasitic trophozoites. Despite its importance for infection, this transformation is not understood at the cellular or molecular levels. In these studies, we report that excystation entails detection of environmental stimuli across the tough extracellular cyst wall leading to highly coordinated physiological, structural, and molecular responses. We found that novel cytoplasmic rearrangements and changes in individual species of mRNA and in cytoplasmic pH occur within the cyst wall in the earliest stage of excystation, in response to conditions modeling cyst ingestion and passage into the human stomach. This suggests that cysts do not contain all the mRNA needed for excystation and emergence and supports ourhypothesisthat external stimuli, including hydrogen ions, may penetrate or be perceived across the cyst wall. In contrast, changes in cyst wall structure or proteins were detected only later in excystation, in the stage that models passage into the human small intestine, where trophozoites can emerge and survive. These findings show that excystation ofG. lambliais a highly complex and active process and provide important insights into its cellular and molecular components.