When exposed to antigens, naive B cells differentiate into different types of effector cells: antibody-producing plasma cells, germinal center cells, or memory cells. Whether an individual naive B cell can produce all of these different cell fates remains unclear. Using a limiting dilution approach, we found that many individual naive B cells produced only one type of effector cell subset, whereas others produced all subsets. The capacity to differentiate into multiple subsets was a characteristic of clonal populations that divided many times and resisted apoptosis, but was independent of isotype switching. Antigen receptor affinity also influenced effector cell differentiation. These findings suggest that diverse effector cell types arise in the primary immune response as a result of heterogeneity in responses by individual naive B cells.