SummaryTwelveBacillus larvaestrains were searched for the presence of extrachromosomal DNA by agarose gel electrophoretic methods. Small plasmid molecules of a uniform size were detected in three of them. The molecular mass was determined at 5·72±SD 0·25 Mdal by electron microscopy. The bacterial cells harbouring plasmid molecules were then tested for resistance to sulphonamides, antibiotics, mercury chloride and cadmium nitrate. No specific resistance or any other function connected with the presence of the plasmid molecules inB. larvaestrains was found. The plasmid DNA, however, has a high stability inB. larvaecells, because the DNA isolates did not change their electrophoretic pattern during a 2-year cultivation of the cells on solid nutrient agar. The plasmid was denoted pBL423 according to theB. larvaestrain in which it was first detected.