Stationary source-free spherical flames ("flame balls") in premixed combustible gases were studied by employing low-gravity fog) environments in a drop tower and an aircraft flying parabolic trajectories to diminish the impact of buoyancy-induced convective flow. Flame balls were found in all mixture families tested when 1) the Lewis number Le of the deficient reactant was sufficiently low and 2) the compositions were sufficiently close to the flammability limits. Probably as a consequence of the reduction in buoyant convection, the flammability limits at pg were significantly more dilute than those at Earth gravity; for example, 3.35% HI vs 4.0% HI in lean Hi-air mixtures. By comparison with analytical and computational models, it is inferred that the phenomenon is probably related to diffusive-thermal effects in low-Li mixtures in conjunction with flame-front curvature and radiative heat losses from the combustion products. The chemical reaction mechanism appears to play no qualitative role. In the aircraft experiments, the gravity levels (« 10-2 £o) were found to cause noticeable motion of flame balls due to buoyancy, which in turn influenced the behavior of flame balls. At these g levels, a new type of transient, nearly cylindrical flame structure, termed "flame strings," was observed.Nomenclature D = mass diffusivity E = overall activation energy of chemical reaction Fr = Froude number g = acceleration of gravity go = Earth gravity Le = Lewis number P = pressure R = radius of flame ball (R = gas constant r = radial coordinate T = temperature r ad = adiabatic combustion temperature of homogeneous mixture T b = adiabatic temperature of flame ball T* -temperature at flame ball surface V = drift velocity of flame ball a. = thermal diffusivity (3 = nondimensional activation energy, = E/(RT fjig = microgravity P -gas density = equivalence ratio 4> c = critical equivalence ratio, = Le fu /Le ox