The fluvial fish fauna of the Gelami and Tinggi Rivers, before and after the construction of a building complex in the adjacent forest, was compared on the basis of a literature survey (covering the period: 1997–2003) and a field survey in 2010. Forty fish species, representing 14 families and 5 orders, were recorded in total, the field survey including new records for Barbonymus gonionotus, Glyptothorax laosensis and Macrognathus maculatus. On the other hand, twelve species, including Osteochilus microcephalus, Luciosoma setigerum, Cyclocheilichthys apogon, Tor tambroides, Acantopsis choirorhynchos, Homaloptera orthogoniata, Clarias teijsmanni, Clarias macrocephalus, Pseudomystus leiacanthus, Mystus nigriceps, Parambassis siamensis and Trichopodus trichopterus, that had been listed previously, were not recorded by the latter. An analysis of the results using McNemar’s chi-squared test indicated that the fish fauna of the rivers has changed significantly following the construction, continuous quantitative monitoring and habitat evaluation being necessary for conservation of future biodiversity levels.