Giardia lamblia, a major cause of intestinal disease worldwide, is a parasitic protozoan that represents the earliest branch of the eukaryotic lineage. Trophozoites, which possess two nuclei but lack mitochondria, peroxisomes and a typical Golgi apparatus, colonize the small intestine of the vertebrate host where they may differentiate into infective cysts. Encystation is a regulated process characterized by the biosynthesis, secretion and formation of a protective extracellular cyst wall. In previous studies, we demonstrated the biogenesis of the Golgi apparatus during encystation and identified two leucine-rich proteins (CWPs), which localize within encystation-specific secretory granules before their incorporation into the cyst wall. Here, we used immunological, biochemical and molecular biological approaches to analyze the expression of BiP/GRP78, an endoplasmic reticulum (ER)-resident chaperone, during the Giardia life cycle. A monoclonal antibody specific for Giardia BiP permitted the visualization of the ER of this protozoan and showed that BiP expression increased simultaneously with the increased expression of CWPs during encystation. However, in contrast to the 140-fold increase in levels of CWP transcripts, the steady-state level of BiP mRNA did not increase during encystation. Furthermore, potent inducers of BiP expression in higher eukaryotic cells, including agents that perturb the ER environment, did not affect BiP expression in Giardia. These results, when considered together with the profound changes that occur in the secretory pathway during Giardia encystation, indicate an important role for this molecular chaperone during the differentiation of this primitive eukaryote.