Pulsed-field gel electrophoresis (PFGE) was applied to the study of the similarity of 55 strains of proteolytic Clostridium botulinum (C. botulinum group I) types A, AB, B, and F. Rare-cutting restriction enzymes ApaI, AscI, MluI, NruI, PmeI, RsrII, SacII, SmaI, and XhoI were tested for their suitability for the cleavage of DNA of five proteolytic C. botulinum strains. Of these enzymes, SacII, followed by SmaI and XhoI, produced the most convenient number of fragments for genetic typing and were selected for analysis of the 55 strains. The proteolytic C. botulinum species was found to be heterogeneous. In the majority of cases, PFGE enabled discrimination between individual strains of proteolytic C. botulinum types A and B. The different toxin types were discriminated at an 86% similarity level with both SacII and SmaI and at an 83% similarity level with XhoI. Despite the high heterogeneity, three clusters at a 95% similarity level consisting of more than three strains of different origin were noted. The strains of types A and B showed higher diversity than the type F organisms which formed a single cluster. According to this survey, PFGE is to be considered a useful tool for molecular epidemiological analysis of proteolytic C. botulinum types A and B. However, epidemiological conclusions based on PFGE data only should be made with discretion, since highly similar PFGE patterns were noticed, especially within the type B strains.The most commonly reported forms of botulism are foodborne botulism, wound botulism, and infant botulism. Foodborne botulism is an intoxication caused by consumption of preformed botulinum neurotoxin, while wound and infant botulism are both infections in which neurotoxin is formed in vivo (13). Proteolytic Clostridium botulinum (C. botulinum group I) is one of six phylogenetically and physiologically distinct clostridia that forms botulinum neurotoxin. It is responsible for a majority of the cases of infant botulism and wound botulism as well as many cases of classical food-borne botulism (1,2,4,16,17). Nonproteolytic C. botulinum (C. botulinum group II) is responsible for most other cases of food-borne botulism (13). The confirmation of food-borne botulism outbreaks has been based on the detection of botulinum toxin or C. botulinum organisms of the same toxin type from both the patient and the suspected food item. In order to be able to confirm the epidemiological relatedness of the strains isolated from patients and from food, a method for genotyping proteolytic C. botulinum isolates is needed. Such a method would also be of utility when investigating cases of wound or infant botulism.Since the first paper on pulsed-field gel electrophoresis (PFGE) typing of C. botulinum type A strains Hall A and 62 A was published (10), no reports on the genetic diversity of strains of proteolytic of C. botulinum as determined by PFGE have been published. With nonproteolytic C. botulinum, a high genetic diversity determined by PFGE and randomly amplified polymorphic DNA (RAPD) analysis has been report...