Dispersal of the water-bloom-forming cyanobacteria Microcystis aeruginosa is an important issue for human and animal health. Passive dispersal of such algae occurs by water currents, wind, and animals (birds, mammals, and insects). Especially waterbirds are thought to be important carriers. In a force-feeding experiment with mallard ducks, fluorescence microscopy of filtered fecal samples showed that the amount of Microcystis in the feces peaked on average 2 hr after feeding. Long-term (10-day) viability of Microcystis cultures reared from feces, as measured from cell counts, decreased with a longer residence time in the duck’s digestive tract; 4 hr residence time was the limit. Cultures from single isolated cells were also established. 1.59% of the cells excreted 2 hr after feeding increased in number. Microcystis was also detected in duck feces using Microcystis-specific 16S rDNA primers until 10 days after feeding. These results suggest that Microcystis aeruginosa can survive passage through the digestive tract intact and retain its proliferative capacity, while the survival rate depends on the retention time. This study was supported by the Environment Research and Technology Development Fund (D-0905) of the Ministry of the Environment, Japan.