Lacustrine algal pisoliths are growing today in many calcareous lakes throughout the midwestern United States. Pisoliths from two Michigan lakes studied are subspherical, and consist of annual couplets of porous and dense concentric laminae surrounding a nucleus. Porous laminae display radial fabric resulting from the growth of algal filaments perpendicular to the nodule surface; these grade outward into dense laminae that contain sparse unoriented filaments. Boundaries between successive couplets are always sharp. Basal flattening, thinning of the porous layer below the center of the pisoliths, and growth in low energy mud-floor environments indicates that these structures form in situ, and are only rarely overturned on the marl platform surfaces.