Harsha Lake, like many other freshwater systems that serve as important recreation and drinking water sources, is suffering from frequent cyanobacterial harmful algal blooms (CyanoHABs). CyanoHABs in Lake Harsha start with Anabaena and Nostoc (N2-fixers) in early Summer and later transit into Microcystis (non-N2 fixers) when N supply is at low levels. To investigate the mechanisms that govern this cyanobacterial community succession, cyanobacteria and heterotrophic bacterial communities were collected from Harsha Lake weekly from June to September 2015 and examined for their metatranscriptomic profiles. Gene transcripts confirmed the dominance of Anabaena and Nostoc and their active N2-fixation in June (N-fixing stage). Meanwhile, heterotrophic bacteria, Proteobacteria and Actinobacteria, were most abundant and mainly expressed genes involved in ATP synthesis. Microcystis and Planktothrix became dominating in July and early August (non-N2 fixing stage) and mainly expressed genes for organic N and P metabolism, DNA repair and secondary metabolites (cyanotoxins) productions. During this non-N fixing stage, Proteobacteria and Actinobacteria significantly increased their relative abundance and expressed more genes for iron and P transport. Proteobacteria and Actinobacteria were still dominant heterotrophic bacteria during CyanoHAB senescence, and they mainly upregulated genes for organic carbon degradation. Further analyses, including weighted gene co-expression network and protein-protein interaction network, will be performed to examine potential interaction between cyanobacteria and heterotrophic bacteria and its role in CyanoHAB succession.