The effects of liquefied petroleum gas (LPG) flaming on soil microbial biomass and soil temperature were studied in a laboratory trial. The trial was arranged to imitate the most common weed flaming practice, non-selective flaming pre-emergence of the crop. Soil samples were taken, without disturbing the soil surface, before the germination of the crop from a field cultivated and sown in the spring. The microbial biomass in the 0–5 mm depth, measured by substrate induced respiration (SIR), was decreased by 19% when the soils were flamed with open flame burners using a flaming intensity of 4600 MJ ha⁻¹. The treatment was quite extreme, corresponding to a propane dose of 100 kg ha⁻¹, while the usual dose in practice is 20–80 kg ha⁻¹. Flaming had little effect on microbial biomass deeper in the soil (5–10 mm). The soil temperature at 5 mm depth was raised by 4.0°C and at 10 mm by 1.2°C. It is concluded that the threat that flaming poses to soil microorganisms is small.