BAHIA SEBASTIAN VIZCAINO consists of a large, shallow embayment partially enclosed on the seaward side by the prominent headland formed by the Sierra Vizcaino, by Isla Natividad, and by Isla Cedros (fig. 1). It is about 65 mi. wide, open on the northwest by a mouth 48 mi. wide, and on the southwest by channels known as Canal de Kellet and Canal de Dewey, 8 and 4 mi. wide respectively. The bottom shows little topographic variation and depths do not exceed 390 ft. except in the vicinity of the north end of Isla Cedros where depths of 510 ft. occur. The sill depth in Kellet Channel is 144 ft., in Dewey Channel 114 ft. Extensive shoals occur in the southeastern part of the bay and extend inland in the form of large lagoons. The prevailing wind is from the northwest (Osorio-Tafall, 1948, map VI) and is unbroken by any coastal formation from Morro de Santo Domingo to Isla Cedros. The mainland on the east and south has an arid climate which supports only a moderately to extremely sparse vegetation of cacti and other xeric shrubs. Isla Cedros on the west has a similar xeric vegetation on its dry, sunny east and south sides, whereas its damp, foggy, northwest side is to a large extent covered with coniferous forest. An extremely prominent boundary running along the main ridge of the northern half of the island separates these vegetation types and their respective climates.2 The circulation in Bahia Vizcaino which creates diverse, localized temperature conditions in various parts of its area is largely responsible for the presence of contrasting juxtaposed marine floras almost as striking as the terrestrial floras on Isla Cedros. A recent study of the relationship of coastal upwelling to the sessile marine vegetation along Pacific Baja California (Dawson, 1951) has shown how the algal flora closely reflects the depressed temperatures resulting from this phenomenon. That survey was based primarily on an uninterrupted temperature transection along almost the entire Pacific Coast of Baja California, but passing to the west of Isla Cedros. It was concerned mainly with the detection of areas of maximum upwelling intensity