Somatic hypermutation in immunoglobulin variable region genes occurs within germinal centers. Here, we describe a subset of germinal center dark zone centroblasts that express only sIgD and have accumulated up to 80 mutations per heavy chain variable region (IgV_Hδ gene). Over half of the hypermutated IgV_Hδ sequences were found to be clonally related. This level of mutation is not observed in either IgV_Hγ transcripts from the same sample or IgV_Hδ transcripts from peripheral blood, suggesting that these cells neither undergo isotype switch nor mature into circulating memory B cells. Optimal growth of these cells in vitro depends on CD40 ligand, T cell cytokines, and a fibroblast stroma, a combination possibly mimicking the dark zone microenvironment. Our hypothesis is that these cells may be sequestered within germinal centers, where their somatic mutation machinery is triggered. The isolation of these hypermutated B cells may represent a critical step for studying both the biology and biochemistry of somatic hypermutation.