Many fungi undergo a morphological transition to filamentous growth in response to limiting nutrient conditions. Constitutively elongated Saccharomyces cerevisiae mutants (elm) have been isolated; the ELM1 gene encodes a putative serine/threonine protein kinase. A novel allele, elm1-15, has been isolated in an S288C-derived strain, which causes a pleiotropic phenotype, including media-specific growth effects, abnormal morphology and altered stress response, in cells that are auxotrophic for tryptophan. elm1-15 trp1 cells cannot use many nitrogen sources, are sensitive to amino acid analogues, have very low general amino acid permease activity and do not accumulate trehalose. In contrast, haploid elm1-15 TRP1 cells grow well in budding form on all media, are stress resistant and overaccumulate trehalose. Several lines of evidence suggest that Elm1 acts on functions related to the RAS/cAMP pathway. Overexpression of Elm1 partially rescues the ts phenotype of cdc25 and cyr1 mutants. Deletion of ELM1 in low PKA activity mutants increased the severity of their phenotypes, and activation of Ras2 decreases the cell elongation phenotype of elm1 mutants. A ‘signal integration’ model for the complex relationship of Elm1 and the RAS/cAMP pathway in controlling morphogenesis in response to nutrients is proposed.