In course of the studies on feeding and nutrition of phytophagous insects, the present investigation was undertaken to determine if the different food values of several host plants for a polyphagous insect could be analyzed by means of measurements of food utilization. Two groups of the last instar larvae of the cabbage armyworm, Mamestra brassicae L., were reared on leaves of beet and sweet-potato plants, respectively, and the data were recorded on dry matter and nitrogen balances.<br>1. The mean nitrogen contents of the beet leaves and sweet-potato leaves were 3.83 per cent and 2.80 per cent, respectively.<br>2. The larvae grew much faster on the beet leaves than on sweet-potato leaves. The fresh and dry weight of pupae grown on the beet leaves and their nitrogen contents are also significantly higher than pupae grown on sweetpotato leaves. These indicate that the beet leaves are the suitable food for the last instar larvae of this insect species as compared with the sweet-potato leaves.<br>3. Although the larvae that were fed on sweetpotato leaves consumed more food during the last instar than did those fed on the beet leaves, the mean value of the daily consumption of food (dry matter consumption per day) was much more on the beet leaves than on the sweet-potato leaves, indicating that the larvae preferred the beet leaves to the sweet-potato leaves. Similar patterns of consumption occurred in regard to nitrogen; the total consumption of nitrogen was much more on the sweet-potato leaves, but the dialy consumption of nitrogen was higher on the beet leaves.<br>4. During the last instar, larvae that were fed on the sweet-potato leaves excreted larger quantities of dry matter and nitrogen than did those fed on beet leaves. The daily excretion of dry matter was also larger when larvae were fed on the sweet-potato leaves. Taking into consideration that the daily consumption of the sweetpotato leaves by larvae was smaller, it is clear that the sweet-potato leaves were utilized by larvae at a low efficiency as compared with the beet leaves.<br>5. Utilization percentage of the dry matter by the larvae was 49 per cent on the beet leaves and 24 per cent on the sweet-potato leaves. This indicates that the beet leaves were utilized by the larvae at a higher efficiency twice as much as the sweet-potato leaves.<br>6. Utilization percentage of nitrogen by the larvae was 32 per cent on the beet leaves and 13 per cent on the sweet-potato leaves. This indicates that the nitrogen compounds in the beet leaves were utilized by the larvae, at a higher efficiency 2.5 times as much as the nitrogen compounds in the sweet-potato leaves.<br>7. According to the results of the present study, it is concluded that beet leaves were a superior food for the last instar larvae of Mamestra brassicae as compared with sweet-potato leaves, and that although difference of the food values between the two plant leaves was partly due to the higher nitrogen content of beet leaves and the higher preference of the insect larvae for beet leaves, the main cause was the difference between utilization percentage of respective leaves by the larvae; particularly the utilization percentage of nitrogen compounds in the leaves was significant. It seems that the nitrogen compounds in beet leaves has superior quality as a nutrient for the last instar larvae.