Pili and fimbriae are filamentous structures on bacteria. Pili are longer, thicker, composed of pilin protein, and involved in motility, DNA transfer, and attachment. Fimbriae are shorter, more numerous, composed of protein subunits, and primarily serve as adhesion structures.

The main difference between fimbriae and pili is that fimbriae are responsible for the attachment of the cell to its substrate whereas pili are responsible for the attachment and horizontal gene transfer during bacterial conjugation. Fimbriae and pili are two filamentous structures that extend from the surface of bacteria.

Pili are fine hair-like microfibers with a thick tube structure, whereas fimbriae are small bristle-like fibres that emerge from the bacterial cell surface. Fimbriae are shorter than pils. Fimbriae occur 200-400 times per cell, while pili occur one to ten times each cell.

Pili are longer and fewer in number than fimbriae. Fimbriae are found in both gram-negative and gram-positive bacteria, while pili are found only in gram-negative bacteria. Fimbriae are composed of fimbilin protein, while pili are composed of pilin protein. Fimbriae are less rigid than pili.