Sporocysts of Isospora endocallimici, a parasite of marmosets, were exposed to minimal essentials medium (MEM) or a trypsin-bile salt solution (TBS) and then fixed and prepared for transmission electron microscopy. Excystation occurred in TBS but not MEM. The sporocyst wall has 2 layers, a thin outer layer (15 to 110 nm thick) and a thick inner layer (65 to 180 nm thick), which is composed of 4 separate curved plates. The outer layer consists of 1 to 3 membranes interspersed with lipid droplets. In the inner layer, a thin layer of material connects the peripheral margins of 2 apposing plates. Immediately beneath this layer, a thin strip of material is interposed between the 2 apposing plates. Ultrastructural changes preparatory to excystation occur primarily in the inner layer of the sporocyst wall. The TBS acts upon the site of apposition between 2 plates causing the interposed strip to swell and separate from the margin of each plate which leads to collapse of the sporocyst. As the sporocyst collapses, the margins of each curved plate curl inward toward the center of the sporocyst.