Trichodesmium sp., isolated from the Great Barrier Reef lagoon, was cultured in artificial seawater media containing a range of Fe concentration. Fe additions stimulated growth, N-2 fixation, cellular chlorophyll a content, light-saturated chlorophyll a-specific gross photosynthetic capacity (P-m(chl a)) and the dark respiration rate (R-d(chl a)). Cell yields only doubled for 9 nM Fe relative to zero added Fe, whereas N-2 fixation increased 11-fold considerably for 450 nM Fe. The results suggest that N-2 fixation of Trichodesmium is more sensitive to Fe limitation than are the cell yields.