The ability to form botulinum neurotoxin is restricted to six phylogenetically and physiologically distinct bacteria (Clostridium botulinum Groups I-IV and some strains of C. baratii and C. butyricum). The botulinum neurotoxin is the most potent toxin known, with as little as 30-100 ng potentially fatal, and is responsible for botulism, a severe neuroparalytic disease that affects humans, animals, and birds. In order to minimize the hazards presented by the botulinum neurotoxin-forming clostridia, it is necessary to extend understanding of the biology of these bacteria. Analyses of recently available genome sequences in conjunction with studies of bacterial physiology are beginning to reveal new and exciting information on the biology of these dangerous bacteria. At the whole organism level, substantial differences between the six botulinum neurotoxin-forming clostridia have been reported. For example, the genomes of proteolytic C. botulinum (C. botulinum Group I) and non-proteolytic C. botulinum (C. botulinum Group II) are highly diverged and show neither synteny nor homology. It has also emerged that the botulinum neurotoxin-forming clostridia are not overtly pathogenic (unlike C. difficile), but saprophytic bacteria that use the neurotoxin to kill a host and create a source of nutrients. One important feature that has contributed to the success of botulinum neurotoxin-forming clostridia is their ability to form highly resistant endospores. The spores, however, also present an opportunity to control these bacteria if escape from lag phase (and hence growth) can be prevented. This is dependent on extending understanding of the biology of these processes. Differences in the genetics and physiology of spore germination in proteolytic C. botulinum and non-proteolytic C. botulinum have been identified. The biological variability in lag phase and its stages has been described for individual spores, and it has been shown that various adverse treatments extend different stages of lag phase. For example, heat treatment primarily extended germination, while incubation at a chilled temperature primarily extended outgrowth. The neurotoxin gene is present within a cluster of associated genes, and can be located on the chromosome, a plasmid or a bacteriophage. Two basic types of neurotoxin cluster have been identified. Evolution of the neurotoxin gene and cluster has occurred independently of the organism, and involved a series of recombination events but is still poorly understood. Factors affecting the regulation of neurotoxin formation also remain poorly understood, and will be the focus of much future research.