When air or helium was injected upwards into water or methanol through long, narrow slots of width 50 less than or equal to w less than or equal to 250 mu m formed in a horizontal surface, small bubbles were generated spontaneously and continuously at an array of discrete sources distributed distance lambda apart along the slot. The dimensionless source spacing was found by experiment to be lambda/w = 17.2(rho(l)/rho(g))(0.16)We(-0.25), where (rho(l)/rho(g)) is the liquid-gas density ratio and We is the modified Weber number for the slot flow. (We = U(s)(2)w rho(l)/sigma, where U-s is the superficial gas velocity in the slot and sigma is the liquid interfacial tension.) The existence and spacing of the sources is attributed to the formation of curvature-dependent Rayleigh-Taylor nodes at the "most dangerous" wavelength on the gas-liquid interface in the slot. A new analysis of the node wavelength is presented for this interface configuration. Bubble volume V-b was found to increase with U-s up to a critical value of U-s given for water by the dimensional equation U-s = 115 rho(g)(-0.12)W(-0.068), beyond which lateral coalescence occurred between neighbouring growth sites with a consequent breakdown of the discrete source array.