Hydrographic data and spatial, temporal, and vertical distributions of Gonyaulax tamarensis are presented from the vicinity of Cape Ann, Massachusetts in summer of 1972. When considered together with available meteorological records, these data provide information on probable causes and development of the September 1972 red tide bloom in the Gulf of Maine. An upwelling of deep water, development of a "seed" population of G. tamarensis during unusually dry weather in August, and heavy rainfall in September are suggested as the three causative factors.