Environmental conditions are described during a bloom of Cochlodinium polykrikoides that occurred between September 12th and November 9th 2012, in Ensenada de La Paz, Gulf of California. At the beginning of the bloom, the number of cells, chlorophyll a, and peridinin was 6.2 x10² cells mL-1, 10.1 mg m-3, and 3.5 mg m-3, respectively; 8.6 x 10³ cells mL-1, 121.2 mg m-3 and 40.2 mg m-3 in their maximum intensity; and 5 cells mL-1, 1.02 mg m-3 of chlorophyll a, and 0.03 mg m-3 of peridinin, at the end of the bloom period. Thermal interval fluctuated between 30.3 and 31.3 °C; salinity varied from 35.2 to 36.8; dissolved O2 ranged from 4.8 to 10.8 mL L-1; saturation percentage was > 200. The values of nitrate, phosphate, and the N:P ratio at the initiation of the bloom were 1.8, 0.9 (µM) and 2.8, during the maximum intensity they were 8.5, 3.4 (µM), and 2.9. At the end of the bloom they were 5.3, 0.6 (µM) and 9.0. Prevailing winds were from the NNE (0.4-5.0 m s-1), and a precipitation of 68 mm was registered previous to the bloom event. As winds diminished < 1.3 m s-1 on to SSE direction, the blooming got more intense. According to this information, we concluded that the NNE winds and continental runoff caused by rain fertilized Ensenada de La Paz waters, created conditions to support growth and survival of C. polykrikoides. Mortality of marine organisms was not observed during this event.