Dry matter and apparent protein digestibility coefficients were determined in Saro­ therodon niloticus fry of mean weight 21 mg maintained on four artificial diets with pro­ tein content ranging from 9.6% (Diet 1) to 30.4% (Diet 4) at salinities of 0°/00, 5°/00 and 10°/00. The total digestibility estimates made using faecal material accumulated through the night did not significantly differ (P > 0.05) from those estimates based on faecal mate­ rial collected during the day time; between feedings. The mean apparent total, protein and energy digestibility of the different groups of fry ranged from 56.7% to 69.1%, 71.7% to 87.1% and 73.6% to 83.9%, respectively. The dry matter digestibility, in all experimen­ tal groups of fry, showed a significant decrease when the dietary protein content changed from 9.6% to 30.4%. Salinity had no significant effect on the digestibility of any of the four experimental diets. In S. niloticus maintained on Diets 3 and 4 at salinities of 5°/00 and 10o/oo the total and protein digestibility showed daily variation. Generally a day or two of high digestibility was followed by a day of low digestibility. The possible utilization of this rhythmicity, whether apparent or real, in aquacultural practices to reduce the feed costs is suggested.