A natural convection heat transfer experiment was conducted in mercury with gas injection in a vertical enclosure heated on one face at constant heat flux and cooled on the opposite face. Nitrogen gas bubbles were injected from a row of hypodermic tubes facing upwards along the bottom of the heated plate. A transverse magnetic field of magnitude 0.07 less-than-or-equal-to B less-than-or-equal-to 0. 50 Tesla was imposed perpendicular to the heated surface. The range of the applied heat flux was 370 less-than-or-equal-to q" less-than-or-equal-to 16 000 W m-2, corresponding to a modified Boussinesq number range of 10(5) less-than-or-equal-to Bo(x)* less-than-or-equal-to 10(9). The gas injection rate range was 0.9 less-than-or-equal-to Q(g) less-than-or-equal-to 9.2 cm3 s-1. Local heat transfer and void measurements were made with thermocouple and double-conductivity probes. Experimental results showed that for low heat flux rates, a small magnetic field intensity (B approximately 0.07 T) significantly reduced the heat transfer coefficient in the presence of gas injection which in the absence of the magnetic field enhanced the heat transfer coefficient two- to three-fold relative to the single-phase result. The decrease was attributed to the suppression of both the laminar convection and the bubble-induced liquid motion. At higher heat fluxes, the decrease in the heat transfer coefficient in the presence of the magnetic field was less significant. Nevertheless, with increasing field intensity the Nusselt number decreased at these higher heat fluxes. An increase in the bubble size and bubble velocity stabilized the continuous decrease in the heat transfer coefficient for field intensities in the range B approximately 0.25-0.35 T. At higher field intensities (B approximately 0.50 T) temperature profiles indicated a conduction-dominated heat transfer mechanism.