TX 75083-3836, U.S.A., fax 01-972-952-9435. AbstractIn steady gas-liquid flow in homogeneous porous media with surfactant present, there is often observed a critical injection velocity or pressure gradient ∇p min at which "weak" or "coarse" foam is abruptly converted into "strong foam," with a reduction of one to two orders of magnitude in total mobility: i.e., "foam generation." Earlier research on foam generation is extended here with extensive data for a variety of porous media, permeabilities, gases (N 2 and CO 2 ), surfactants, and temperatures. For bead-and sandpacks, ∇p min scales like (1/k), where k is permeability, over 2½ orders of magnitude in k; for consolidated media, the relation is more complex. For dense-CO 2 foam, ∇p min exists but can be less than 1 psi/ft. If pressure drop, rather than flow rates, is fixed, one observes an unstable regime between stable "strong" and "coarse" foam regimes; in the unstable regime ∇p is nonuniform in space or variable in time. Results are interpreted in terms of the theory of foam mobilization at a critical pressure gradient.