In Saccharomyces cerevisiae, a network of signal transduction pathways governs the switch from yeast-type growth to pseudohyphal and invasive growth that occurs in response to nutrient limitation. Important elements of this network have been identified, including nutrient signal receptors, GTP-binding proteins, components of the pheromone-dependent MAP kinase cascade and several transcription factors. However, the structural and functional mapping of these pathways is far from complete. Here, we present data regarding three genes, MSN1/MSS10, MSS11 and MUC1/FLO11, which form an essential part of the signal transduction network establishing invasive growth. Both MSN1 and MSS11 are involved in the co-regulation of starch degradation and invasive growth. Msn1p and Mss11p act downstream of Mep2p and Ras2p and regulate the transcription of both STA2 and MUC1. We show that MUC1 mediates the effect of Msn1p and Mss11p on invasive growth. In addition, our results suggest that the activity of Msn1p is independent of the invasive growth MAP kinase cascade, but that Mss11p is required for the activation of pseudohyphal and invasive growth by Ste12p. We also show that starch metabolism in S. cerevisiae is subject to regulation by components of the MAP kinase cascade.