The residual activity of permethrin on the canvas of an experimental tent, used by nomadic refugees in a malarious area in northern Pakistan, was assessed. A permethrin emulsion sprayed on the inner sheet of a pitched double-fly tent (0.5 g/m2) had an effective residue for at least 6 months in bioassays using the local malaria vectors, Anopheles stephensi and Anopheles culcifacies. However, a high concentration of permethrin (2 g/m2) sprayed on the inner surface of the outer tent, simulating single-fly tents, decomposed within 2 months. These findings were confirmed with chromatographic analysis. Under the conditions found in the study area, the shortened residual effect most likely resulted from photodecomposition, not previously reported to affect operational performance of this insecticide.