The genus Nodularia [Mertens in Juergens] Bornet et Flahault 1888 is considered with regard to its ecology, distribution, physiology and taxonomy. Ecology was studied by periodic sampling of organisms and monitoring environmental parameters in four saline ponds in the British Columbia interior, as well as by sampling a variety of brackish, marine and other inland saline water bodies. Nodularia was collected in habitats of alkaline pH 8.2 - 10.0) and medium to high dissolved salts (4 - 60 °/00 ) and is associated with a specific group of eury-oo haline bluegreen algae. However, environments in which it grows are usually dominated by species of green algae, with Nodularia only occasionally becoming dominant. The distribution of Nodularia was examined by reference to herbarium collections and literature reports. Although many bluegreen algae are considered cosmopolitan, Nodularia has a temperate and subtropical distribution with few reports from polar or tropical latitudes. The growth and physiology was investigated using 16 isolates from Canada, United States, Great Britain and Australia. Isolates were grown in a wide range of conditions of light, temperature, pH and dissolved salts in defined media. Generally, the highest growth rates were obtained with relatively high light intensity (550 - 600 ft-c), temperature of 25 - 30 C, pH near 10 and dissolved salts of 5-30 °/00- Highest rates of growth were obtained with NO3- as the nitrogen source as opposed to NH4+ or urea. Nodularia was found to fix nitrogen in pure culture. Nodularia was established in 1822 by Mertens, but not validily published until 1888 by Bornet and Flahault. At present there have been 28 taxa described. The validity of these and the variability in nature and of isolates in culture in a variety of chemical and physical conditions was considered. An evaluation of the reliability of the taxonomic characteristics indicates that sheath and akinete characteristics are variable while vegetative cell shape, heterocyst location and aspects of akinete formation are more stable characteristics. On the basis of the observations reported, all the described taxa can be included within two species, N. spumigena [Mertens in Juergens] Bornet et Flahault 1888 or N. harveyana (Thw.) Thuret 1875, or placed in other genera.