Xylella fastidiosa possesses both type I and type IV pilli at the same cell pole. By use of a microfluidic device, the speed of twitching movement by wild-type cells on a glass surface against the flow direction of media was measured as 0.86 (standard error [SE1, 0.04) mu m min(-1). A type I pilus mutant (fimA) moved six times faster (4.85 [SE, 0.27] mu m min(-1)) and a pilY1 mutant moved three times slower (0.28 [SE, 0.031 mu m min(-1)) than wild-type cells. Type I pili slow the rate movement, while the putative type IV pilus protein PilY1 is likely important for attachment to surfaces.