1. Four salmonid and four cool-water fish species were tested to determine their ability to moisturize their digesta.
2. After the fish were fed, they were sacrificed, the gut contents were removed and water content was determined.
3. The digesta of the salmonids contained the least water (63–72%) and those of largemouth bass the most (78%).
4. We conclude that there are distinct and significant differences between species and genera in the ability of fish to moisturize their digesta. The potential significance of this finding is discussed.