An active form of Cry4A is a heterodimer of the 20- and 45-kDa fragments that are derived from the 130-kDa Cry4A protoxin. To investigate the function of these two fragments, several deletion mutants were constructed and expressed in E.coli as the GST (glutathione-S-transferase) fusion proteins. The results of the bioassay against Culex pipiens larvae showed that the interaction of two fragments of Cry4A was necessary for the toxicity, and that the C-terminal 67 amino acids of the 20-kDa fragment corresponding to the helices α4 and α5 were involved in determining the insecticidal activity. Surprisingly the lack of helix α5 did not affect the toxicity to C. pipiens, suggesting that the role of helix α5 of Cry4A was different from that postulated in the case of Cry4A toxins.