Abstract: Several recombinant proteins with N- and C- terminal deletions and binding domain modifications of insecticidal Cry3Aa toxin were prepared and examined in laboratory bioassays. Tenebrionid larvae 2 ± 0.5 days after eclosion were individually exposed to an array of seven different Cry3Aa toxin derivates in a wide concentration range. Lethal and sublethal effects (weight gain, number of moultings, length of development) were monitored. The most pronounced effect on Tenebrio molitor larvae was revealed for the 73 kDa non-modified Cry3Aa (TOXst) protoxin and a similar protoxin derivate with a modified peptidase binding site (TOX_cath). For both toxins statistically significant lethal and sublethal effects were documented. Bioassays performed with Tribolium castaneum over the entire larval stage confirmed that this species is highly insensitive to Cry3Aa even at concentrations of 2500 mg/kg. As the tested toxin effects on the tenebrionid storage pests was low, Leptinotarsa decemlineata, known as highly susceptible to Cry3Aa, was included in our list of tested species. Recently, a leaf disk bioassay procedure has been developed and will be used for comparison of relative efficiency of the newly produced modified recombinant Cry3Aa toxin derivates.