The effect of inoculation density level for three Azolla species namely A. microphylla, A. filiculoides and A. pinnata was investigated under greenhouse conditions in term of senescence associated with Azolla fronds overcrowding. In this respect three inoculum levels, i.e., 0.2 kg, 1.0 kg and 2.0 kg m^-2 were tested.
 Overcrowding influence on Azolla was assessed in terms of frond yellowing, nitrogenase activity and heterocyst frequency. Frond discoloration of A. microphylla, A. filiculoides was observed after only 7 days in higher inocula level (1.0 and 2.0 kg m^-2). However, in the lower level of inocula 0.2 kg m^-2 the discoloration was observed after 15 days. However, A. pinnata exhibited frond discoloration on day 10 for 0.2 kg and on day 5 for higher inoculum levels. The nitrogenase activity increased significantly on 7 days at less crowded condition in the cultures inoculated with 0.2 kg m^-2 for all the tested species. At higher inoculum levels, the nitrogenase activity declined on the third week, after which very low enzyme activity, overcrowding, and extensive senescence were noticed at all inoculurn levels. Azolla plants maintained in the dark for 12 hrs prior to beginning of dark incubation period of 3 hrs showed an almost zero nitrogen-fixing activity compared to plants exposed to continuous light. Heterocyst formation on the 12^th leaf from the tip increased up to the seventh day in the three species at 0.2 kg m^-2and in A. pinnata only at 1.0 kg m^-2 inoculum level. There was an apparent gradually decline in the heterocyst count in the three species at higher inoculum levels throughout the culture period.