Aim: This study aimed to quantify changes in fire severity resulting from the invasion of Australia's tropical savannas by the African grass Andropogon gayanus Kunth. (gamba grass). Location: Mesic savannas of the Northern Territory, Australia. Method: Byram's fire-line intensity (If), fuel load and architecture, and two post-fire indicators of fire intensity - scorch height (SH) and char height (CH) of woody vegetation - were determined for fires in native grass savanna and A. gayanus invaded savanna. Leaf scorch is the height at which the fire's radiant heat browns leaf tissue, and leaf char is the height that radiant heat blackens or consumes leaf tissue and provides an indirect measure of flame height. These data, and 5 years of similar data collected from the Kapalga Fire Project in Kakadu National Park, were used to develop empirical relationships between If and the post-fire indices of fire intensity. Results: A relationship between A. gayanus If and SH could not be developed because complete canopy scorch occurred in most A. gayanus fires, even at low If. In contrast, A. gayanus If was strongly correlated with CH. This empirical relationship was substantially different from that for native grass fires. For a given If, there was a significantly greater CH in invaded sites. This increase in radiant heat is attributable to the increased biomass (mean 3.6 t ha-1 in native grasses compared to 11.6 t ha-1 in A. gayanus) and height (approximately 0.5 m in native grasses compared to 4 m in A. gayanus) of the standing fine fuel. Main conclusion: Andropogon gayanus invasion resulted in substantial changes in fire behaviour. This has important regional implications owing to the current (10,000-15,000 km2) and predicted (380,000 km2) area of invasion and the negative consequences for the native savanna biota that has evolved with frequent but relatively low-intensity fire. © 2010 Blackwell Publishing Ltd.