The entomocidal delta-endotoxins CytA and CytB produced by Bacillus thuringiensis (Bt) subspecies israelensis and kyushuensis respectively showed a similar level of toxicity to mosquito larvae but were not toxic to the larvae of the lepidopteran Manduca sexta. CytA and CytB are also similar in sequence, predicted secondary structure and alpha-helical content, the only obvious difference being a C-terminal fifteen residue 'tail' on CytB. Investigations of the function, if any, of the CytB C-terminal 'tail' showed that this delta-endotoxin is highly expressed and forms inclusions in an acrylstalliferous Bt mutant without the aid of the 20 kDa 'helper' protein from Bt subspecies israelensis which is essential for CytA inclusion formation. After proteinase K treatment, CytA and CytB were processed to virtually the same points in a sequence alignment and were equally haemolytic in vitro. However, the results suggested that unprocessed CytB differs from unprocessed CytA in that the former is not haemolytic.