Mitogen-activated protein kinase (MAPK) cascades are conserved signalling modules that regulate responses to diverse extracellular stimuli, developmental cues and environmental stresses (reviewed in refs 1-3). A MAPK is phosphorylated and activated by a MAPK kinase (MAPKK), which is activated by an upstream protein kinase, such as Raf, Mos or a MAPKK kinase. Ste7, a MAPKK in the yeast Saccharomyces cerevisiae, is required for two developmental pathways : mating(4) and invasive (filamentous) growth(5,6). Kss1 and Fus3, the MAPK targets of Ste7, are required for mating(7,8), but their role in invasive growth has been unclear.