Single-cell algae (the so-called symbiotic zooxanthellae) living interstitially in a marine jellyfish and a sea anemone were isolated from the host tissue and cultured in vitro. In culture the zooxanthellae gave rise to motile forms whose morphology clearly marked them as dinoflagellates. Isolation procedures and some nutrient requirements for the in vitro culture of such zooxanthellae are described, as well as methods used for serial passage.