In the photic zone of the open sea, procaryotic cells with an ultrastructure typical of chroococcoid cyanobacteria and similar sized populations of autofluorescent bacteria (0.5–1.0 × 1.0 µm) were observed at concentrations usually between 103–104 cells per ml. The ubiquitous ultrastructural type was an orange‐autofluorescing, phycoerythrin‐containing cyanobacterium amenable to culture. Two other morphological types which have not been cultured occur sporadically in larger concentrations, one apparently favoring deeper water. These phototrophs can account for about 20% of the total bacterioplankton biomass and from 6 to 15% of the total microbial plankton. These cells may play a significant role in oceanic primary productivity.