No measurable differences in Trichodesmium nitrogenase activity were observed between colonies collected by diving and incubated under ultra-clean conditions compared with those collected and incubated using standard techniques. Measurements were made in the northeastern Caribbean Sea, near the Bahama Islands and in the Sargasso Sea. Surprisingly, mean rates of ethylene production were high relative to most previous in situ measurements on Trichodesmium . The calculated cellular N doubling times (via N 2 fixation) ranged from 1.13 days in the northeastern Caribbean Sea, 1.48 days in the Sargasso Sea to 1.8 days near the Bahama Islands. A comparison of these doubling times with those in the literature illustrates the high variability in rate of N 2 fixation by Trichodesmium . From this study, we conclude that the often observed slow rates of N 2 fixation are valid. Populations of Trichodesmium can probably remain within the water column at low growth rates via gas vesicles, which keep the colony suspended, and low grazing rates by herbivores.