Pieris brassicaeis one of the most serious pests of cultivated crucifers in the world. The present experiment is intended to determine the proteolytic digestion of fifth instar larvae and the effects of four cruciferous hosts toP. brassicae nutritional physiology. Brassica oleracea var. botrytis, Brassica oleracea var. capitata, Brassica oleracea var. italica and Brassica oleracea var. viridis were assessed under laboratory conditions at 25 ± 2°C, 60 ± 5% RH and a photoperiod of 16: 8 (L: D) h. The maximum total proteolytic activity in the midgut extract was observed at pH 11, suggesting the presence of serine protease. The lowest rate of tryptic activity and both the highest general proteolytic (0.11 ± 0.01 Umg-1) and chymotryptic activities (0.45 ± 0.002 Umg-1) were observed in B. oleracea var. viridis. The proteolytic activity of P. brassicae on this host plant indicates that B. oleracea var. viridisis not a suitable host for the pest likely due to the presence of some secondary chemicals or proteinaceous protease inhibitors.