Teleost fish are the most primitive bony vertebrates that contain B cells; thus, comparative analysis of teleost naive/mature B cells and plasma cells can provide helpful evidence for understanding the evolution paradigms of these two B-cell subpopulations in vertebrates. In this study, we developed monoclonal antibody against grass carp IgM and identified two different IgM(+) cell subsets: IgM(+) lymphocytes (Lym), resembling naive/mature B cells, and IgM(+) myeloid cells (Mye), resembling plasma cells. Like plasma cells in mammals, the size of IgM(+) Mye is significantly larger than that of IgM(+) Lym, as revealed by flow cytometric analysis and transmission electron microscopy. The IgM(+) Mye were further verified as plasma cells because they showed gene expression patterns similar with those of human plasma cells and a great capacity to secrete IgM. Like mammalian IgM(+) and IgA(+) plasma cells, not IgG(+) plasma cells, grass carp IgM(+) Mye also expressed membrane immunoglobulins, a feature conserved in IgM(+) plasma cells in vertebrates. Furthermore, recombinant CD4OL or IL-21 alone could induce the plasma cell generation and IgM secretion, while the combination of CD4OL and IL-21 had greater effect on IgM secretion, but not on plasma cell generation. This study fills an important gap in the knowledge of plasma cells in teleost fish and provides critical insights into the conserved evolution of IgM(+) plasma cells in vertebrates.