The Fermi surface (FS) of ferromagnetic cobalt has been investigated using the de Haas-van Alphen effect. Measurements were made at temperatures down to 1.1 °K and magnetic fields up to 78 kOe using a low-frequency field-modulation technique. Since cobalt is ferromagnetic, the dHvA oscillations are periodic in B-1 rather than H-1, as in nonmagnetic metals. Orientation studies of the dHvA frequencies in the (101¯0) plane show two sets of frequencies. The lower set [(1-2) × 106 G] has two branches and may be tentatively assigned to pockets of U symmetry of the spin-down FS. The higher frequencies [(3-5) × 106 G], which can be observed up to 39° away from the hexad axis, were attributed to the existence of a neck at the Γ point of the spin-up FS. Agreement with band calculations is found if some modifications are made to the published FS models, mainly by reducing the exchange energy to fit the value found by photoemission studies. dHvA frequencies of (11-14) × 106 G were also observed, but are still unexplained.