Abstract The rate of food intake, the percentage digestibility, and the efficiency of conversion of ingested and digested food to body matter during the fifth instar of Prodenia eridania was determined for 18 different plants, representing 13 families. Ten of the plants were very efficiently converted to body matter. Some plants did not support optimal larval growth because of any one or a combination of the following factors: low digestibility, low efficiency of conversion, and low consumption rate. Digestibility ranged from 36 per cent for a poor to 76 per cent for a good food. Efficiency of conversion of digested material to body matter ranged from 16 per cent for a poor to 56 per cent for a good food. Other factors which appear to affect the overall efficiency of a particular plant are water content, protein content, and possibly fibre content. There appears to be in a good food plant an inverse correlation between food consumption and efficiency of utilization. The efficiency by which digested food can be converted into body material is 50 per cent and higher in Prodenia and several other lepidopterous larvae, on a dry weight basis. This degree of efficiency is far higher than in the most efficient food converters among higher animals, viz. the chick or pig, where comparable figures are of the order of 10 to 25 per cent.