Bacillus thuringiensis produces a floating biofilm in glass tubes when cultured in static conditions. This biofilm is composed of a pellicle that covers the whole liquid surface and a ring that sticks to the tubes walls. A massive recruitment of planktonic bacteria, located in the culture medium beneath the biofilm, occurs during biofilm formation. Using a built-up device and molecular biology techniques, we were able to quantify the recruited population in the biofilm ring, and to determine the molecular mechanisms involved in this process. Screening of a transposon insertion mutant library for severe recruitment defects led to the discovery of the gene bip (for Biofilm Integration Protein). A prevalence study showed that bip is found in more than 50% of B. thuringiensis strains isolated from Lebanese soils. Deletion of bip by allelic exchange gravely affects the recruitment capacity and complementation of the mutant strain restores the wild type phenotype. Located on a 8.5 kb plasmid, bip encodes for a 21 kDa protein which displays two C-terminal transmembrane domains and a signal peptide, suggesting that Bip might be exported and anchored on the cell wall. By using immunofluorescence methods we confirmed that Bip is located at the bacterial surface. We therefore expect Bip to interacts with molecular components of the biofilm, which we are currently looking for. We expect that this study will shed new light on how incoming pathogenic bacteria integrate host resident biofilms.