The ground-state properties of ferromagnetic fcc Co have been calculated in the local-spin-density approximation for exchange and correlation. Large magnetic effects on the lattice constant and bulk modulus are found. In addition, the calculations show that, as in fcc Fe, fcc Co has two magnetic states, one with low volume and low moment, and the other with higher volume and higher moment. It is unlikely that effects due to the low-moment state in Co are directly observable. © 1978.