The effect of formaldehyde treatment on the solubility of fish meal in M.NaC1 solution was investigated. Subsequently the variation in solubility of fish meals obtained from various sources was examined. A series of metabolic trials was then conducted with Merino sheep in order to determine to what extent fish meal proteins with different solubilities are utilized by animals on low quality roughage rations and the same rations supplemented with various amounts of maize meal and maize starch. The addition of coarse maize meal to the rations resulted in an increase in apparent protein digestibility and nitrogen retention with increasing levels of supplementation. In the case of maize starch. however, increasing levels of supplementation soon reached a stage when the values for these parameters started to decrease. The point of decrease was reached at a much lower level of supplementation in the case of the more insoluble proteins. It is possible that the utilization of fish meal proteins was determined by a relationship between their solubility or apparent digestibility and the rate of passage of digesta through the digestive tract. The treatment of fish meal with formaldehyde decreased its digestibility. The micro-organisms in the rumen may, under certain conditions even have suffered from a nitrogen deficiency, which could have lead to a suppression of organic matter digestibility and feed intake.