ABSTRACT The hydrogen (H ₂ ) cycle associated with the dinitrogen (N ₂ ) fixation process was studied in laboratory cultures of the marine cyanobacterium Crocosphaera watsonii . The rates of H ₂ production and acetylene (C ₂ H ₂ ) reduction were continuously measured over the diel cycle with simultaneous measurements of fast repetition rate fluorometry and dissolved oxygen. The maximum rate of H ₂ production was coincident with the maximum rates of C ₂ H ₂ reduction. Theoretical stoichiometry for N ₂ fixation predicts an equimolar ratio of H ₂ produced to N ₂ fixed. However, the maximum rate of net H ₂ production observed was 0.09 nmol H ₂ μg chlorophyll a (chl a ) ⁻¹ h ⁻¹ compared to the N ₂ fixation rate of 5.5 nmol N ₂ μg chl a ⁻¹ h ⁻¹ , with an H ₂ production/N ₂ fixation ratio of 0.02. The 50-fold discrepancy between expected and observed rates of H ₂ production was hypothesized to be a result of H ₂ reassimilation by uptake hydrogenase. This was confirmed by the addition of carbon monoxide (CO), a potent inhibitor of hydrogenase, which increased net H ₂ production rates ∼40-fold to a maximum rate of 3.5 nmol H ₂ μg chl a ⁻¹ h ⁻¹ . We conclude that the reassimilation of H ₂ by C. watsonii is highly efficient (>98%) and hypothesize that the tight coupling between H ₂ production and consumption is a consequence of fixing N ₂ at nighttime using a finite pool of respiratory carbon and electrons acquired from daytime solar energy capture. The H ₂ cycle provides unique insight into N ₂ fixation and associated metabolic processes in C. watsonii .