Throughout tropical and subtropical seas, Trichodesmium spp. contribute significantly to marine fixation of atmospheric di-nitrogen and influence the global carbon cycle. We suggest that dissolved inorganic phosphate (DIP) availability has a predominant role in controlling Trichodesmium spp. biomass. From experimental work carried out on cruises in the SW Pacific Ocean, and by re-analysing previous data, we have defined a critical level of DIP needed for single filaments of Trichodesmium spp. to grow. Thus, seasonal variations in DIP availability could control Trichodesmium spp. growth and decay. As this critical level is below the detection limit of classical DIP measurements obtained during oceanic cruises, we suggest a re-evaluation of the phosphate availability in the oligotrophic ocean in order to determine what ultimately controls di-nitrogen fixation in the sea.