Abstract: Mat formation in the bakers' yeast Saccharomyces cerevisiae is a surface-associated phenomenon in which yeast cells spread over the surface of a low-density agar petri plate as a complex film. This spreading growth occurs by sliding motility and is dependent on the adhesion protein (adhesin) Flo11p. In order to identify molecular pathways that govern mat formation, whole-genome transcriptional profiling was used to compare cells growing as a mat to cells growing in a suspension culture (planktonic cells). This analysis revealed that S. cerevisiae upregulates a subset of genes in response to growth on a surface. These genes included the INO1 gene, which encodes the myo-inositol-1-phosphate synthase, which carries out the rate-limiting step in inositol biosynthesis. Further inquiry revealed that a transcription factor that controls INO1 expression, called Opi1p, participates in the regulation of mat formation. Opi1p appears to modulate mat formation by influencing the expression of FLO11. The opi1Delta mutant was found to exhibit reduced FLO11 levels. Consequently, the opi1Delta mutant perturbs the FLO11-dependent phenotype of invasive growth. The opi1Delta mutant's defects in mat formation and invasive growth are dependent on the transcriptional activator Ino2p. These results indicate that Opi1p affects mat formation and invasive growth by participating in the regulation of FLO11.