Shephard's synthetic sea water medium for Acetabularia (Shephard, 1970) was compared with natural sea water-based media. Cells in natural sea water supplemented with nitrate, phosphate, mineral micronutrients and vitamins grew faster and formed caps sooner than those grown in the synthetic medium. This difference was not overcome by the addition of soil–water extract, other trace elements, higher levels of nitrate or phosphate, or a greater number of vitamins. On the other hand, sea water had no effect on maximum cell length, percentage of cells forming caps, the time between cap initiation and cyst formation, or the time needed for gamete release. Since three species of Acetabularia have completed their life-cycle in Shephard's medium, and two species have been maintained in it for several generations, the sea water seems to be providing a stimulatory factor rather than an essential nutrient. The variability of natural sea water and the ease of preparing large quantities of sterile synthetic medium make the latter very attractive for raising large numbers of cells.