Plasmid profiles in fifteen toxigenic and six nontoxigenic strains of Clostridium botulinum type A were investigated. The electrophoretic patterns of nucleic acids from the bacterial lysates were shown to vary in relation to the mode of bacterial cells lysis and the stage of microbial population growth. Two toxigenic strains harbour a unique 3.9 MD plasmid, five strains have a plasmid of about 6 Md and seven other strains carry an 11.5 Md plasmid. The nontoxigenic derivatives of two toxigenic strains harbouring a 6 Md plasmid have lost this extrachromosomal element. The analysis of the derivatives has not indicated the 6 Md plasmid to be connected with bacteriocin production or antibiotic resistance in Clostridium botulinum. Nontoxigenic variants of three other strains possess an 11.5 Md plasmid similar to their toxigenic parents' one. One of the studied toxigenic strains harboured no plasmids, as well as its nontoxigenic derivative. The functions of all studied plasmids remain unknown. Physical maps of the 11.5 and 6 Md plasmids were constructed on the basis of restriction analysis. The obtained results can be used for identification and differentiation of Clostridium botulinum strains and in further molecular biological experiments with this organism.