Description / Table of Contents: Summary 1. Members of the epiphytic communities ofZostera, Enteromorpha, Ulva and other marine macrophytes occurring in North Sea Harbor, Southampton, New York are being isolated in gnotobiotic culture as part of a broader program of studies on the trophic dynamics and mineral cycling of the community. 2. Aseptic samples are taken in the field, by means of sterile forceps, and inoculated into 30 ml of sterile sea water from the same station. The samples are refrigerated and brought as soon as possible to a nearby field laboratory where they are inoculated into a series of (26) differential media. These media (Lee et al. 1966), originally based on the formulation ofPintner & Provasoli (1957), have been modified each year as more experience with the nutrition of the isolates from the community is gained. Incubation is usually 1–3 weeks at 15–25° C. Many diatoms, chlorophytes, bacteria, and yeasts can be isolated axenically from clones grown on solidified initial isolation media. 3. Other organisms are isolated from agnotobiotic media by: (a) streaking onto solidified media; (b) aseptic washing of individual organisms in 9-hole spot plates; (c) inoculation into antibiotic-containing media; and (d) combination of several or all of the above techniques. Much of the isolation and transfer is performed under a stereoscopic microscope enclosed in a Germ Free microscope glove-box. Tracer feeding techniques are used to identify food for gnotobiotic culture of herbivores (Lee et al. 1966). 4. Among the goals of our studies are: (a) the characterization of the microbial community structure of epiphytic communities in comparable unpolluted and polluted areas and study changes in time and space; (b) isolation of the key members of the microbial community in axenic culture and study of their physiological ecology as well as responses to stress conditions; (c) using the above cultures, to construct meaningful small defined microcosms and through experimental manipulations to define the subtle variables which regulate community structure. 5. To this end we now have in continuous gnotobiotic cultureEnteromorpha intestinalis, Ulva lactuca, approximately 70 species of salt marsh diatoms, chrysophytes, dinoflagellates and chlorophytes, six species of foraminifera, and three species of nematodes. In agnotobiotic culture are additional species of harpactacoid copepods, ostracods, amphipods, flagellates, and ciliates associated with epiphytic communities.