Quantum yields, Phi, for the production of the formyl radical, HCO, in the photolysis of glyoxal were determined at 85 wavelengths, lambda, in the range of 290-420 nm at pressures between 50 and 550 Torr (N-2) at 298 K using pulsed-laser photolysis combined with cavity ring-down spectroscopy detection of HCO. HCO quantum yields were parametrized using a Stern-Volmer analysis to obtain extrapolated zero-pressure NCO quantum yields, Phi(0)(lambda), and values for the ratio of the rate coefficients for quenching and dissociation, k(q)/k(d)(lambda), at each wavelength. Phi(0)(lambda) varied smoothly with wavelength with a maximum value of similar to 1.8 in the range 300-385 nm with values decreasing to near 0 at 420 nm and 0.4 at 290 nm. k(q)/k(d)(lambda) was measurable at nearly all photolysis wavelengths and is well-represented by the relationship k(q)/k(d)(lambda) = (2.3 x 10(-20)) +(1.5 x 10(-19)) exp(-0.4 Delta E) (cm(3) molecule(-1)) where Delta E = ((28 571/lambda) - 72.5) (kcal mol(-1)), lambda is the photolysis wavelength (nm), and 72.5 kcal mol(-1) is the threshold for glyoxal photodissociation. Differences in our HCO quantum yield wavelength- and pressure-dependence with previous studies are discussed. The present HCO quantum yield data are appropriate for use in atmospheric model calculations, and revised wavelength-dependent photolysis branching ratios for the production of 2HCO, H2CO + O-2 and H-2 + 2CO at atmospheric pressure are presented.