Seasonal distribution, nitrogen fixation and primary productivity of Trichodesmium species were intermittently studied in the coastal waters of Tanzania. Samples were collected in 1975/6, 1980, 1993/4, 1994/5 and 1998/9. Four colony forming species were found, i.e. T. erythraeum, T. tenue, T. thiebautii and one unidentified Trichodesmium sp. while T. contortum was rarely encountered, and only as individual trichomes. T. erythraeum was most abundant, although other Trichodesmiumspecies dominated during particular periods of the year. The occurrence of Trichodesmium showed a consistent seasonal pattern. High Trichodesmium biomass was promoted by the NE monsoon (December–April) while it was low or absent during the SE monsoon (June–October). The biomass was highest at the surface especially during calm weather periods. The NE monsoon was characterized by elevated rainfall, temperature and nitrate concentrations while salinity, light intensity and turbidity tended to decrease. Phosphate concentrations did not show systematic variations with season. The rate of nitrogen fixation by Trichodesmium species in surface waters was 1.8 1.6 pmol N trichome–1 h–1 giving an average N2 fixation of 42.7 mmol N m–3y–1. The mean rate of carbon fixation was 1.15 0.3 ng C trichomes–1 h–1 in the upper 5 m depth. It is estimated that Trichodesmium contributes about 0.03–20% of the total CO2 fixation in the coastal surface waters during the SE and NE monsoon, respectively.