Several bluegreen algae capable of fixing nitrogen were identified and isolated from Portonovo marine environments. The most common forms are Ambaena sp., Aphanocapsa sp., Nodularia sp. and Trichodesmium sp. For planktonic bluegreen algae the estimated nitrogen fixation rates were 0.504 mg N/m³/d in estuary, 0.238 mg N/mVd in coastal waters and 0.336 mg N/m°/d in mangrove waters. This could contribute 3.5%, 1.6% and 2.2% of nitrogen requirements of phytoplankton of the respective environments (assuming an average daily productive rate of 100 mg C/m'/d and a C: N ratio of 6). Nitrogen fixation by benthic bluegreen algae were 3.22 and 3.32 mg N/m'/d for estuarine and mangrove environments. The contribution of benthic bluegreen algae to the nitrogen requirement of benthic macroalgae could be about 0.72% on the basis of a net production value of 2700 mg C/m'/d. The epiphytic algae could fix about 0.968 mg N/m'/d in estuary and 1.347 mg N/m'/d In mangrove waters. The contribution by epiphytic algae to the nitrogen demands of macroalgae was only 0.2% and 0.3 % in the estuary and mangrove waters.