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Humpback grouper (Cromileptes altivelis) is one of superior commodity that are easily cultivated in the fisheries sector in Indonesia. But, there is one of the difficulties in humpback grouper cultivation is Viral Nervous Necrosis (VNN) infection. The existence of VNN can be transmitted vertically and horizontally, according to water quality and natural feed (plankton) consumed by groupers. Therefore, this study conducted an observation of the factors thought to affect the presence of VNN seen from the identified plankton. This study used descriptive method. Water sampling, plankton and fish was carried out 3 times with a one-week interval in floating net cages in the Gulf of Sendang Biru Beach, Malang Regency. The results obtained by 5 phylum of phytoplankton, there are Chlorophyta division (3 genera), Phragmophyta (1 genus), Diatom (2 genera), Cyanophyta (1 genus) and Bacillariophyta or Chrysophyta (2 genera). Zooplankton consists of 4 phylum, there are Arthropoda (7 genera), Mollusca (1 genus), Tracheophyta (1 genus) and Rotifera (1 genus). Whereas, plankton that were found in the stomach of groupers mostly Bacilariophyta and a small portion of Cyanophyta and Phragmophyta. Based on the results of plankton identification, showed that plankton was not infected with Viral Nervous Necrosis (VNN) and indicated the infecction of Viral Nervous Necrosis (VNN) occurs not because of the plankton factor in the floating net cages but vertically which is the spread from the parent to the larvae.