cGMP-dependent protein kinases are key intracellular transducers of cell signaling. We identified a novel dominant mutation in the C. elegans egl-4 cGMP-dependent protein kinase (PKG) and show that this mutation causes increased normal gene activity although it is associated with a reduced EGL-4 protein level. Prior phenotypic analyses of this gain-of-function mutant demonstrated a reduced longevity and a reduced feeding behavior when the animals were left unperturbed. We characterize several additional phenotypes caused by increased gene activity of egl-4. These phenotypes include a small body size, reduced locomotion in the presence of food, a pale intestine, increased intestinal fat storage, and a decreased propensity to form dauer larvae. The multiple phenotypes of egl-4 dominant mutants are consistent with an instructive signaling role of PKG to control many aspects of animal physiology. This is among the first reported gain-of-function mutations in this enzyme of central physiological importance. In a genetic screen we have identified extragenic suppressors of this gain-of-function mutant. Thus, this mutant promises to be a useful tool for identifying downstream targets of PKG.