Abstract: Cyanobacterial bloom and fish mortality in ponds and lakes are common in Bangladesh but the relationship between the two has been rarely studied. In April 2002, mass mortality of fishes occurred in a farmer’s pond of Mymensingh, Bangladesh. At the time of fish kill, a massive bloom of cyanobacteria (210.5×103 cells ml‾1) was found in the pond. Microscopic study showed the dominance of Aphanizomenon flos-aquae (L.) Ralfs and Microcystis aeruginosa, Kütz representing 60.91 and 33.98% of the total phytoplankton population, respectively. Fish species affected were silver carp, Hypophthalmichthys molitrix (Valenciennes), tilapia Oreochromis niloticus (L.), catla Catla catla (Hamilton Buchanon) and common carp Cyprinus carpio (L.) and among these silver carp, tilapia and catla were severely affected. The gills of dead fishes were pale-white and the microscopic analysis of the gill squashed water showed the presence of large number of A. flos-aquae and M. aeruginosa cells. Gut content analysis of dead silver carp, tilapia and catla revealed the presence of 49.12, 43.5 and 19.35% A. flos-aquae and 30.37, 38.32 and 9.75% M. aeruginosa, respectively among the total consumed phytoplankton. On the day of fish mortality, high concentration of PO4-P (9.5 mg l‾1), high water temperature (310C) and low dissolved oxygen concentration (0.95 mg l‾1) were found. The fish mortality was possibly caused either by oxygen deficiency or toxins secreted by cyanobacteria or by combination of both.