The aggregation phenotypes Agr(+) and Agr(-) of Bacillus thuringiensis subsp. israelensis are correlated with a conjugation-like plasmid transfer and characterized by the formation of aggregates when the bacteria are socialized during exponential growth. We present evidence for the association of the Agr(+) phenotype with the presence of the large (135-MDa) self-transmissible plasmid pXO16.