The zinc finger regions of the Aspergillus nidulans PacC transcription factor, mediating regulation of gene expression by ambient pH, and the Saccharomyces cerevisiae Rim1p transcription factor, mediating control of meiosis and invasiveness, are homologous and both transcription factors undergo proteolytic processing of the C-terminus for conversion to the functional form. In both cases, functioning of a signal transduction pathway involving several gene products is a necessary prerequisite for processing. We now show that the Aspergillus Pall pH signal transduction component is homologous to the Saccharomyces Rim9p meiotic signal transduction component throughout a region containing four hydrophobic, putative membrane-spanning segments. This suggests that Pall might be a membrane sensor for ambient pH. Deletion of the pall gene established that the less extreme phenotype of pall mutations compared with mutations in the other five genes of the pH signalling pathway is a general feature of pall mutations.