The horizontal distribution of four species of planktonic algae (Asterionella formosa, Tabellaria flocculosa, Chlorella vulgaris, and Chlamydomonas moewussii) growing during summer in surface lake water is described. All showed considerable variation in standing crop. By application of a pattern analysis technique, these surface distributions of algae were shown to be contagious with patterns of horizontal distribution which differed between species in dimensions and intensity. More extensive analysis of the distribution of A. formosa at contrasting times of year showed the existence of pattern in horizontal distribution to be a recurrent phenomenon.