The complex cytopathology of mitochondrial diseases is usually attributed to insufficient ATP. AMP-activated protein kinase (AMPK) is a highly sensitive cellular energy sensor that is stimulated by ATP-depleting stresses. By antisense-inhibiting chaperonin 60 expression, we produced mitochondrially diseased strains with gene dose-dependent defects in phototaxis, growth, and multicellular morphogenesis. Mitochondrial disease was phenocopied in a gene dose-dependent manner by overexpressing a constitutively active AMPK α subunit (AMPKαT). The aberrant phenotypes in mitochondrially diseased strains were suppressed completely by antisense-inhibiting AMPKα expression. Phagocytosis and macropinocytosis, although energy consuming, were unaffected by mitochondrial disease and AMPKα expression levels. Consistent with the role of AMPK in energy homeostasis, mitochondrial “mass” and ATP levels were reduced by AMPKα antisense inhibition and increased by AMPKαT overexpression, but they were near normal in mitochondrially diseased cells. We also found that 5-aminoimidazole-4-carboxamide-1-β-d-ribofuranoside, a pharmacological AMPK activator in mammalian cells, mimics mitochondrial disease in impairing Dictyostelium phototaxis and that AMPKα antisense-inhibited cells were resistant to this effect. The results show that diverse cytopathologies in Dictyostelium mitochondrial disease are caused by chronic AMPK signaling not by insufficient ATP.