An electron microscopical study has been performed on quenched bainite-austenite interfaces in 55SiMnMo steel.From these observations,it is conclu-ded that the bainiticferrite-austenite is more closely associated with theWidmanstatten ferrite-austenite interface than with the lath martensite-aus-tenite interface,which is expected to posess defects of screw character.