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The extensive clearing of mangroves over the last 50 years has caused many areas to vanish worldwide, or they are now degraded sites with secondary vegetation, or rehabilitated areas with modified species and community characteristics. Baseline ecological studies of undisturbed mangrove ecosystems are required to support management and conservation objectives. A detailed study of two transects in the virtually pristine Sematan mangrove forest, Sarawak, Malaysia was conducted in November 1999. Both transects extended inland from the shoreline. A quantitative study of the mangrove vegetation (trees, saplings, seedlings and associated species) was recorded from 100 m2 plots from each of the transects, to include species diversity, density and basal area. In total, 28 mangrove species were recorded, including 18 considered true mangroves. Xylocarpus granatum was the dominant species in terms of frequency and basal area. Species similarity matrices for the trees, saplings and seedlings were positively correlated, suggesting that young stages tend to occur near their parent tree. The mangrove associated species were negatively correlated to the mangrove saplings, which could mean they prefer different habitat types or they inhibit each other. The environmental variables, salinity, pH, redox potential, temperature, soil moisture, soil carbon, nitrogen and sulphur content, leaf litter and tidal inundation were also recorded for each plot. Correlations were made between these environmental variables and the mangrove vegetation. Soil moisture content, pH and the presence of Thalassina anomala mud lobster activity were important factors in the distribution of the mangrove vegetation. Soil nitrogen content and the number of decaying leaves were correlated to the mangrove associates similarity matrix. The landward sites with high organic decomposition provide a good nutrient status for the mangrove associates. The baseline data from this study confirm that Sarawak supports a diverse mangrove forest community and can provide a valuable comparison with other more degraded or rehabilitated sites in Southeast Asia. [Copyright &y& Elsevier]