The extracellular biofilm matrix often contains a network of amyloid fibers which, in the human opportunistic pathogen Bacillus cereus, includes the two homologous proteins TasA and CalY. We show here, in the closely related entomopathogenic species Bacillus thuringiensis, that CalY also displays a second function. In the early stationary phase of planktonic cultures, CalY was located at the bacterial cell-surface, as shown by immunodetection. Deletion of calY revealed that this protein plays a major role in adhesion to HeLa epithelial cells, to the insect Galleria mellonella hemocytes and in the bacterial virulence against larvae of this insect, suggesting that CalY is a cell-surface adhesin. In mid-stationary phase and in biofilms, the location of CalY shifted from the cell surface to the extracellular medium, where it was found as fibers. The transcription study and the deletion of sipW suggested that CalY change of location is due to a delayed activity of the SipW signal peptidase. Using purified CalY, we found that the protein polymerization occurred only in the presence of cell-surface components. CalY is, therefore, a bifunctional protein, which switches from a cell-surface adhesin activity in early stationary phase, to the production of fibers in mid-stationary phase and in biofilms.