P20 is a small chaperone-like protein encoded by a cry11A operon in Bacillus thuringiensis subsp. israelensis (Bti); it is essential to Cyt1Aa expression. In this report, the gene P20 was transformed into wild strains of subsp. kurstaki to raise the yield of crystal proteins. As a result, larger crystals were produced by the transformant than by the wild control, and most were in the form of a big bipyramid (average 2.4 mum long); some were irregular because of too high expression, while the spores turned out to be small spheroids unlike the long rods in the wild strains. SDS-PAGE analysis confirmed that Cry1A protoxin production was doubled by P20, but no increase of Cry2A production was observed. Besides, P20 caused obvious changes not only in bacterial morphology, but in the sporulation process as well. Further investigation discovered that a serious degradation happened to Cry1A in vivo of the wild strains, and reconfirmed that P20 was effective in preventing the degradation. Our results suggest that P20 is useful in engineered strain construction with enhanced protein expression.