SummaryChloroplast development requires coordinated expression of both nuclear‐ and chloroplast‐encoded genes. To better understand the roles played by nuclear‐encoded chloroplast proteins in chloroplast biogenesis, we isolated an Arabidopsis mutant, egy1‐1, which has a dual phenotype, reduced chlorophyll accumulation and abnormal hypocotyl gravicurvature. Subsequent map‐based cloning and DNA sequencing of the mutant gene revealed a 10‐bp deletion in an EGY1 gene, which encodes a 59‐kDa metalloprotease that contains eight trans‐membrane domains at its C‐terminus, and carries out β‐casein degradation in an ATP‐independent manner. EGY1 protein accumulation varies between tissue types, being most prominent in leaf and stem tissues, and is responsive to light and ethylene. EGY1‐GFP hybrid proteins are localized in the chloroplast. egy1 mutant chloroplasts had reduced granal thylakoids and poorly developed lamellae networks. Furthermore, the accumulation of chlorophyll a/b binding proteins of the light‐harvesting complexes I and II (Lhca and Lhcb) are significantly decreased in three separate loss‐of‐function egy1 mutants. Taken together, these results suggest that EGY1 metalloprotease is required for chloroplast development and, hence, a defective EGY1 gene has pleiotropic effects both on chloroplast development and on ethylene‐dependent gravitropism of light‐grown hypocotyls.