The number of peripheral blood B lymphocytes doubles during acute exercise, but the phenotypic composition of this response remains unknown. In two independent exercise studies, using complimentary phenotyping strategies, we investigated the mobilisation patterns of distinct B cell subsets. In study one, nine healthy males (mean ± SD age: 22.1 ± 3.4 years) completed a continuous cycling bout at 80% MAX for 20 mins. In study two, seven healthy experienced cyclists (mean ± SD age: 29.9 ± 4.7 years) completed a 30 min cycling trial at a workload corresponding to +5% of the individual blood lactate threshold. In study one, CD3−CD19+ B cell subsets were classified into immature (CD27−CD10+), naïve (CD27−CD10−), memory (CD27+CD38−), plasma cells / plasmablasts (CD27+CD38+) and finally, recently purported ‘B1’ cells (CD27+CD43+CD69−). In study two, CD20+ B cells were classified into immature (CD27−IgD−), naïve (CD27−IgD+), and IgM+/IgG+/IgA+ memory cells (CD27+IgD−). Total B cells exhibited a mean increase of 88% (study one) and 60% (study two) during exercise. In both studies, immature cells displayed the greatest increase, followed by memory cells, then naïve cells (study one: immature 130% > mature 105% > naïve 84%; study two: immature 110% > mature 56% > naïve 38%). Our findings show that, unlike T cells and NK cells, B cell mobilisation is not driven by effector status, and, for the first time, that B cell mobilisation during exercise is comprised of immature CD27− IgD−/CD10+ cells.