Combustion wind tunnels are often used to investigate the propagation of free-moving fires through solid-phase fuels, typically standardised artificial' fuel beds. However, the results of such studies are difficult to apply directly to wildland fire situations primarily due to the disparity between the generally uniform artificial fuel and the heterogeneous fuel found in nature. To explore the feasibility of using heterogeneous natural' fuel beds in subsequent combustion wind tunnel experiments, this study quantified the variability in forward rate of fire spread resulting from the use of heterogeneous fuel beds in a combustion wind tunnel under a given set of burning conditions. The experiment assessed the effects of fuel type and air speed, and controlled for the effects of fuel moisture content, fuel load and fuel particle size. It was found that the variability in rate of spread increased with its mean, but the overall residual variance (s2e <0.025, s.e. 0.011) was low compared with the effects of air speed and fuel type. This demonstrates that heterogeneous fuel beds can be used in combustion wind tunnel experiments without introducing a large degree of variability. © IAWF 2016