A. gemmatalis (velvetbean caterpillar) is the major pest that attacks soybean (Glycine max) foliage as primary host. This study analyzed the development and the activity of digestive proteases of A. gemmatalis reared on diets supplemented with different concentrations of two different proteins. Casein (C) or soybean (S) proteins were added to artificial diets in concentration ranging from 1 to 4 %. When casein was added, soybean was not, and vice-versa: C0-S1, C0-S2, C0-S3, C0-S4, C1-S0, C2-S0, C3-S0 and C4-S0. The larvae were reared in one of these diets during all the larval period. Larvae were weighed on day 8 and 12. The larval phase duration and pupae viability were also analyzed. The total proteolytic activity and trypsin-like activities were investigated in enzymatic material obtained from midguts of the last instar larvae. All dietary groups presented significant increase of weight from day 8 to 12. However, it is possible to verify that, except for C0-S2 x C2-S0 comparisons, all groups reared on casein protein presented lower weight than the larvae reared on the same protein concentration of soybean. Besides, it was observed that the higher soybean concentration the higher the corporal weight. This effect was not observed to casein protein. These results agree with the higher larval phase duration presented by larvae reared on casein protein. In relation to pupae viability, the results demonstrated similarity among the groups regardless the quality and quantity of proteins added to the diet. The total proteolytic activity revealed differences not significant among the groups. However, this difference was significant for the trypsin-like activity. On the whole, the gain of weight of the caterpillars reared with soybean protein was greater than the gain of weight of reared with casein. These results suggest that soybean protein presented a greater positive nutritional action than the casein, what may reflect in a smaller duration of larval phase. Although the trypsin-like activity from midgut of larvae of the different nutritional groups presented significant differences, it did not follow the specific effects of concentration and/or quality of protein.