Ten strains of the blue-green alga Aphanizomenon flos-aquae were obtained in unialgal culture by isolating single colonies from samples of waterbloom collected in south central Saskatchewan and eastern Ontario. Two strains from Saskatchewan produced large, ribbon-like colonies up to 30 mm long and 5 to 6 mm wide in a medium containing two different preparations of soil extract. Both strains produced typical small colonies in medium without soil extract or with other soil extract preparations. The large colonies produced by these strains were similar to those observed in plankton from south central Saskatchewan, Oregon, southern Germany, and Russia. Eight strains from Ontario produced only typical small colonies under identical or comparable conditions. After a prolonged period of growth in soil extract medium, all strains became partially or completely non-colonial. Soil extract was not essential for growth. Differences in the colony habit of the 10 strains in soil extract medium or of colonies observed in plankton samples could not be correlated with differences in cellular dimensions or appearance. Colony habit appears to depend on genetic and nutritional factors which may involve the associated bacterial flora as well as the alga.