A laboratory culture of Ulva lactuca L. purified with antibiotics in 1958 can assume different morphologies. In synthetic media of the ASP type it grows as a pin-cushion composed of uniseriate branching filaments. Only occasionally some finger-like germlings appear which, after a few millimeters growth, disintegrate and originate the branched uniseriate filaments. Many strains of marine bacteria, isolated from Ulva and other seaweeds collected in nature or from laboratory cultures, when grown together with this axenic Ulva, induced growth of long thin Enteromorpha-like tubes (active bacteria). Other bacterial strains have no visible morphological effect on the Ulva (inactive). Combinations of several active and inactive bacterial strains result in flat ribbons which can be wide or narrow and with distromatic portions intercalated between tubular portions. No combination has so far induced the axenic Ulva to produce the typical expanded foliose thallus. Bacterial filtrates are inactive; syntrophic growth seems indispensable. The bacterial strains lose the ability to induce morphogenic effects on the axenic Ulva when grown for several transfers in organic marine media and regain it by growing with Ulva for several transfers in synthetic mineral media. Each partner appears therefore to be dependent on the metabolites produced by the other. The perpetuation of a wide morphological versatility of this laboratory strain of Ulva after 15 yr maintenance under under artificial conditions emphasizes the phylogenetic importance of this versatility.