Intracellular symbioses between cnidarians and dinoflagellates from the genus Symbiodinium are widespread throughout the marine environment. These associations are ecologically significant, especially in tropical waters where symbiotic interactions between corals and Symbiodinium culminate in the formation of limestone reefs. This thesis focuses on cellular and molecular aspects of the symbiosis, specifically the initiation of the symbiosis and characterization of a host gene, sym32, that is believed to function in the symbiosis. Sym32 was originally identified as a differentially expressed protein in symbiotic vs. aposymbiotic individuals of the sea anemone, Anthopleura elegantissima. Based on its deduced amino acid sequence, sym32 belongs to a family of cell adhesion proteins that play roles in cell recognition in a diverse array of organisms. Chapter 2 examines the process by which a new cnidarian host acquires its first symbionts. Larvae of the scleractinian coral Fungia scutaria, which are initially aposymbiotic, acquired symbionts while feeding. Symbionts that entered the larval gastric cavity with food were subsequently taken into host gastrodermal cells by phagocytosis. Chapter 3 describes immunolocalization of sym32 in A. elegantissima tentacles. In aposymbiotic tentacles, sym32 was localized to vesicles within the host gastrodermal cells. Symbiotic tentacles lacked sym32-containing vesicles. Instead, sym32 was present among the membranes that enclose the symbionts within host cells. Western blots of proteins from Symbiodinium revealed a 45/48kD doublet that cross-reacts with anti-sym32 antiserum. This suggests that homologous proteins are expressed in both host (32kD) and symbiont (45/48 kD). Chapter 4 describes the effects of environmental factors on expression of host sym32. Aposymbiotic and symbiotic anemones maintained in continual darkness for 3 weeks experienced a dramatic decline in sym32 protein levels, relative to anemones maintained on a 12:12 h light:dark cycle. This suggests that light plays a major role in regulating sym32. Exposure of anemones to elevated temperatures for 2 days in the dark caused a mild bleaching response (expulsion of symbionts from the host), but did not affect the levels of sym32 protein. Chapter 5 examines the role of sym32 during the infection process, using antibody interference techniques. F. scutaria larvae and symbionts incubated in sym32 antiserum during the infection process experienced a decline in infection rates. Further, symbionts that were incorporated into host gastroderm appeared to be degenerating in antiserum treatments, but appeared to be healthy in preimmune controls. / Graduation date: 2003