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Zabrotes subfasciatus larvae possess three α‐amylase isoforms as determined by in gel assays following SDS‐PAGE. The two minor isoforms present lower electrophoretic mobility than the major form, and seem to occur as a heterodimer. When developed inside Vigna unguiculata (cowpea) seeds, fourth instar larvae have minor quantities of the slow-migrating forms, but when reared on seeds of Phaseolus vulgaris(common bean) or Phaseolus lunatus, the two slow-migrating forms are expressed in higher amounts, while activity of the major form was independent of the host seed. Larvae developing inside cowpea seeds at the beginning of the fourth instar were fed on flour from cotyledons of cowpea or common bean. Larvae fed on the common bean flour started to express the dimer in higher amounts when compared with the control larvae fed on cowpea flour. In an attempt to correlate differences between starch granules and the induction of α‐amylases, a detailed study on the digestive process of the granules was conducted. Incorporation of purified starch granules into artificial diets did not induce the two minor α‐amylases. The in vitro hydrolysis rates of purified granules and the pattern of dextrins liberated by the different α‐amylases were similar for the two legume species. The starch granules enter the midgut extensively damaged, which may facilitate the access to the more susceptible parts of the granules to enzymatic attack. © 2001 Elsevier Science Ltd. All rights reserved.