Stable isotopes were used to evaluate water utilization for three tree species (Cleistanthus sumatranus, Lasiococca comberi var. pseudoverticillata and Celtis wightii) growing on karst soils in the seasonal tropical rainforest of Xishuangbanna, Southwest China. The soil on the site is only 40 cm thick but was underlain by thick limestone bedrock with large water storage capacity. Dense radiational fog occurred frequently during the pronounced dry season. Isotope composition of water in xylem, soil and fog were analysed, and soil and stem porous water content were measured concurrently to determine the proportion of water deriving from fog, soil and bedrock by adult trees and seedlings at the peak of the dry season. Results indicated that C. wightii seedlings suffered stronger water stress and greatly relied on available moisture from fog (23.8% on average). This suggests that fog water was an important source for seedling growth during the pronounced dry season. In contrast, adult trees had little or negligible water uptake from fog moisture (3–7% on average) but derived around 63–85% of their water from bedrock sources. Water held within bedrock was essential for meeting plant transpiration requirements over the dry season drought. Copyright © 2013 John Wiley & Sons, Ltd.