Abstract The inhibitory potential of purified protease and α-amylase isoinhibitors of sweet potato and taro (5 accessions each) on the digestive enzymes of four major root crop pests viz, Araecerus fasciculatus, Sitophilus oryzae, Cylas formicarius elegantulus and Tribolium castaneum was studied under in vitro conditions. Wide differences in inhibitory potential were noticed among the isoinhibitors of a single accession as well as among the same isoinhibitor of the different accessions. The isoinhibitor SPAI1 from Kanhangad was inhibitory to all the four insect α-amylases (25–58% inhibition), while only 0.8–15% inhibition was exerted by the isoinhibitor SPAI1 from S 1195. Very high inhibition of A. fasciculatus and C. formicarius elegantulus α-amylases (73–94%) was caused by isoinhibitors SPAI2 and SPAI4 from the sweet potato accession S 56-2. Cylas formicarius elegantulus α-amylases were inhibited to a greater extent by the taro α-amylase inhibitor. Among the four insect proteases, those from A. fasciculatus and T. castaneum were not appreciably inhibited by the protease isoinhibitors of sweet potato and taro. The S. oryzae protease was inhibited by 51% by isoinhibitor SPI2, while only 3% inhibition was caused by isoinhibitor SPI4. The selective inhibitory potential of the isoinhibitors of sweet potato and taro on the digestive enzymes of root crop pests could be exploited for making transgenic plants with improved resistance against major pests.