The conclusion that over 25% of global primary production depends on direct algal/bacterial symbiosis involving vitamin B12 [Croft et al., (2005) Algae acquire vitamin B12 through a symbiotic relationship with bacteria. Nature, 438, 90–93] is patently false, for it is based on a misconception of the probable level of the vitamin B12 requirement in marine pelagic algae. A review of the various published attempts at measuring this requirement suggests that it is likely to be so low that oceanic and coastal concentrations of the vitamin would usually be sufficient to sustain the populations that occur without the assistance of direct algal/bacterial symbiosis. The levels measured are discussed in relation to method (batch or continuous culture) and protocols used. Requirement values considered by the author to be acceptable range from 0.1 to 0.3 pM for the vitamin growth saturation constant (KS) and from 30 to 100 μL algal biomass pmol−1 vitamin for the yield.