The filamentous heterocystous cyanobacterium, Richelia intracellularis Schmidt have been suggested to be among the most important nitrogen fixing cyanobacteria in tropical and subtropical waters, but they are less studied in the tropical Western Indian Ocean waters. The spatial and temporal distribution of this cyanobacterium was studied in the coastal waters of Zanzibar and Dar es Salaam, Tanzania. The Richelia sp. was found as an endosymbiont within five species of diatom Rhizosolenia spp., two Hemiaulus species and rarely as epiphyte to Chaetocerus spp. or freely in waters. The morphology and sizes of Richelia sp. did not show big variations but the number of vegetative cells per filament ranged from 4 to 14 cells. Abundance of Richelia sp. ranged from zero in some samples to mean maximum of 428 ± 105 filaments l −1 . The diatom–diazotroph associations were found throughout the year peaking during southeast monsoon. Blooms (up to 1554 filament l -1) of Richelia sp. were recorded in July to August 1993. High rates of nitrogen fixation occurred during northeast monsoon with maximum value (2.75 ± 0.03 nmol N h -1 l -1) in February corresponding to high numbers of Trichodemium spp. Lower values were obtained during southeast monsoon with the lowest value (0.03 ± 0.005 nmol N h -1 l -1) recorded in August when the number and type of cyanobacteria including Richelia sp. was very low. The results clearly indicate that Richelia sp. may contribute significantly to the productivity of the studied waters through its ability t