Mortality in submature channel catfish, Ictalurus punctatus (Rafinesque), caught in the Mississippi River and stocked in Iowa streams and ponds, led to this investigation in 1961-1963. The major findings can be summarized as follows:

The mean concentration of lactic acid in the blood of unexercised channel catfish varied from 2.3 to 7.6 mg/100 ml of whole blood. That of fish subjected individually to forced exercise varied from 21.1 to 44.1 mg/100 ml after 5 minutes of exercise and from 59.1 to 67.6 mg/100 ml after 15 minutes of exercise. A lower concentration of lactic acid, 28.2 mg/100 ml, for fish exercised 15 minutes in a group indicates that fish exercised in a group were not subjected to as strenuous muscular activity as were those exercised individually. Mean lactic acid values were usually greater at higher temperatures, both for unexercised and exercised fish.