The redshift z_abs=3.08 absorption complex towards the z_em=3.12 quasar 0420-388 has high HI column density components with heavy element abundances ~1/10 solar. A low-redshift component of the complex has HI column density ~10^18 cm^-2, and some of the higher order Lyman lines are clean enough to permit a measurement of the deuterium column density for this component. However, while the putative deuterium column density is fairly well-determined, the HI column density is very uncertain so, if the deuterium identification is correct, the D/H ratio for this component could have any value <~2x10^-5. The DI/OI ratio is much better constrained, and has a value ~2. If the OI/HI ratio is constant throughout the complex, then D/H~2x10^-4.