An objectionable “earthy-musty” flavor is frequently found in intensively-cultured catfish which renders the fish unmarketable. Fifty-gram channel catfish were held in 150-liter stainless-steel tanks containing dense masses of either Symploca muscorum or Oscillatoria tenuis to determine the capability of these geosmin-producing blue-green algae to impart this type of offflavor in the fish. Within 2 days the fish had acquired distinct earthy-musty flavor similar to the odor of the algae. Flavor intensity reached a maximum at 10 days. When fish which had been in the algae tanks for 14 days were transferred to flowing, charcoal-filtered water, the flavor disappeared after 10 days. Fish held in tanks containing algae-free filtrate from the algae culture tanks acquired the off-flavor but at a slower rate than the fish in the culture tanks. This indicated that the fish could absorb the flavor compounds without ingesting the algae, possibly across the gill membranes.