The detailed investigation of a typical biostrome of the blue-green colonial alga Aphanothece Packardii along the shore of the Great Salt Lake, at Promontory Point, Utah, has shown a distinct morphological zonation of the algal growth. The latter consists of four zones from the lake to the land: subparallel festooned ridges; tonguelike festooned ridges; composite rings and flat-topped mounds; small isolated mounds. Detailed cross sections through the biostrome have shown that the morphological zones merely reproduce and frequently exaggerate an underlying topography eroded in firm argillaceous and oolitic sands. The conclusion is reached that these colonial algae have no characteristic growth pattern of their own but have developed on several types of positive areas separating a system of erosional channels trending at right angles to the shore line.