Diazotrophic activities were monitored in vitro in three non-heterocystous cyanobacteria belonging to families Leptolyngbyaceae, Merismopediaceae, and Chroococcaceae. The nitrogenase activity was determined chromatographically using the acetylene reduction assay. The study yielded unexpected results as these were also found to generate methane (CH4). The highest observed rate of nitrogen (N2) fixation by Leptolyngbya sp. colony was 10.17 ± 2.86 µmole cm−2 h−1 or 35.08 ± 3.22 nmole mg−1 dry-wt h−1; Synechocystis sp. culture exhibited a value of 8.60 ± 0.28 µmole ml−1 h−1 or 1.76 ± 0.37 nmole cell−1 h−1; and Gloeocapsa sp. culture demonstrated 6.39 ± 0.95 µmole ml−1 h−1 or 0.29 ± 0.53 nmole cell−1 h−1 of N2 fixed. Highest CH4 production from the three cultures was 235.81 ± 17.19 µmole cm−2 h1 or 61.23 ± 13.08 nmole mg−1 dry-wt h−1 (Leptolyngbya sp.); 55.09 ± 9.66 µmole ml−1 h−1 or 11.29 ± 1.22 nmole cell−1 h−1 (Synechocystis sp.), and 39.85 ± 4.05 µmole ml−1 h−1 or 1.81 ± 0.46 nmole cell−1 h−1 (Gloeocapsa sp.). The interdependency of the N2 fixation and CH4 generation was plotted as a linear regression: Y = 6.4115x + 7.5132, p << 0.05, r2 = 0.9842 proving the emission of CH4 from cultures of N2 fixing cyanobacteria to be statistically significant as well.