Two Chesapeake Bay isolates of unicellular cyanobacteria belonging to the genus Synechococcus are described. Unicellular cyanobacteria are suspected to be important primary producers in estuarine and marine waters. One isolate (P-11-16) fluoresces red and forms green colonies. The other isolate (P-11-17) fluoresces orange and forms red colonies. Their ultrastructure is very similar to other isolates of Synechococcus except that spinae are formed and are attached to an outer wall layer not found in previously described species. The spinae are straight-walled cylinders, not flared at the base, are 44.0-65.0 nm in diameter, and range up to 2.7 micrometer in length. Substructure of the spinae wall consists of either material organized into stacks of rings or a strand of material helically coiled at a low (1-6 degrees) angle. Such material yielded a 6.0-9.2 nm cross-banding periodicity. Substructure of the rings or strand appeared to consist of bar-shaped, repeating units as seen in negatively stained material. Other procaryotic cell types with spinae, which were isolated from unincubated, natural seawater, are described.