Three forms of algal mats : subaerial, emersed and submersed, and their species composition, structure and stratification were studied for a period of twelve months (1996-1997) in an irrigation canal at Al-Kharj, Saudi Arabia. The development of the mats is a continuous process in a gradient fashion from the outlet of a piedmont cave-lake to the end of the canal. The growth maxima occurred at the first weir, which is subject to fast water-current and spray-wetted zone. Mats developed as discrete associations, recognizable by their colour and shape. Hard lithified, soft and floppy, and thin crust types of mats developed in their respective zones and comprised separate biotic communities. Ecological factors such as the nature of the microhabitat, water-chemistry, fluctuation in water-flow, depth of water column, splashing and temperature may have influenced their formation. The emersed mats with Phormidium tenue were predominant from November to February, whereas the subaerial mats above with Tolypothrix distorta adhere to the canal wall as a permanent feature. The subaerial mat has stromatolite-like morphology and is considered a nuisance to the water and drainage maintenance.