This work is a study of the biofouling potential of hydrophobic membranes in direct contact membrane distillation (DCMD) with coastal seawater as a feed. The effects of temperature regime, hydrodynamic conditions (cross flow and parallel flow) and water quality were investigated. Highly hydrophobic polypropylene (PP) hollow fiber membranes with fluorosilicone coating were used. The rate of biofilm development on the membranes was investigated including the effect of the fibers' position in the module (next to entrance and next to exit). In most of the experiments there was little or no drop in flux when operated at 40 degrees C with cross flow DCMD. However, under these conditions, SEM images show the presence of microorganisms on the membranes. The fibers placed at the entrance of the module were less fouled compared to membranes near the exit of the module and this most likely reflects the difference in temperature between entrance and exit. Modules tested in temperature cycles between 40 degrees C and 70 degrees C were less fouled than those run at a constant temperature of 40 degrees C. (C) 2011 Elsevier B.V. All rights reserved.