The organic layer (i.e., litter and duff) can represent the majority of the fuels in southeastern forest ecosystems. Measurements of organic layer mass (or loading) are critical for estimating fuel consumption in wildland fires and resulting fire effects, including smoke emissions and tree mortality. Managers generally only have time to measure the depth of the litter and duff layers. To convert depth measurements to mass, bulk density numbers are required for each layer. Previous field research to quantify organic layer bulk densities in the southeastern region of the United States has been generally confined to a small number of samples in very specific forest types. In this study, litter and duff depths and bulk densities were quantified from 2280 samples collected in 5 forest types (slash pine, loblolly pine, hardwoods, shortleaf pine, and mixed pine and hardwoods) across 7 states in the southeastern United States. Average site litter depths ranged from 0.88 to 2.19 inches, with an average depth of 1.44 inches for all forest types. Litter bulk densities ranged from 0.95 to 3.29 t/ac/in, with an average of 1.91 t/ac/in. Duff depths ranged from 0.32 to 2.41 inches; average duff depth was 0.91 inches. Duff bulk densities ranged from 3.12 to 11.02 t/ac/in; average bulk density was 6.10 t/ac/in. Evaluating the data by forest type, the average litter and duff depths and bulk densities for pine sites were 1.44 and 0.91 inches and 2.04 and 6.41 t/ac/in. Average litter and duff depths and bulk densities for hardwood sites were 1.42 and 0.88 inches and 1.38 and 4.84 t/ac/in. This study improved estimates of bulk density values for use in the Fuel Characteristic Classification System, consumption and fire effects software, including Consume 3.0 and the First Order Fire Effects, and the new DMM600 Duff Moisture Meter. They also provide improved estimates of carbon stores and of smoke produced during wildland fires in southern forests.