Macroautophagy (herein autophagy) is an evolutionarily conserved process, requiring the gene ATG5, by which cells degrade cytoplasmic constituents and organelles. Here we show that ATG5 is required for efficient B cell development and for the maintenance of B-1 a B cell numbers. Deletion of ATG5 in B lymphocytes using Cre-LoxP technology or repopulation of irradiated mice with ATG5(-/-) fetal liver progenitors resulted in a dramatic reduction in B-1 B cells in the peritoneum. ATG5(-/-) progenitors exhibited a significant defect in B cell development at the pro- to pre-B cell transition, although a proportion of pre-B cells survived to populate the periphery. Inefficient B cell development in the bone marrow was associated with increased cell death, indicating that ATG5 is important for B cell survival during development. In addition, B-la B cells require ATG5 for their maintenance in the periphery. We conclude that ATG5 is differentially required at discrete stages of development in distinct, but closely related, cell lineages.