Soil CO2 efflux rates were measured in a eucalypt open forest in a tropical savanna of northern Australia, with a portable closed chamber and CO2 gas analyser. Both abiotic (soil temperature and water content) and biotic (litterfall and fine-root growth) factors that may influence soil CO2 efflux were examined. Daytime rates of soil CO2 efflux rate were consistently higher than nocturnal values. Maximal rates occurred during late afternoons when soil temperatures were also maximal and minimum values were recorded during the early morning (0400-0800 hours). Average soil CO2 efflux was 5.37 mol m-2 s-1 (range 3.5-6.7 mol m-2 s-1) during the wet season and declined to 2.20 mol m-2 s-1 (range 1.2-3.6 mol m-2 s-1) during the dry season. The amount of carbon released from soil was 14.3 t ha-1 year-1, with approximately 70% released during the wet season and 30% during the dry season. The rate of efflux was correlated with soil moisture content and soil temperature only during the wet season, when root growth and respiration were high. During the dry season there was no correlation with soil temperature. These results are discussed in relation to the carbon balance of tropical savannas.