Water exchange between the Gulf of California and the Pacifi c has a signifi cant vertical component. Surface (0 -200 m) gulf water fl ows out into the Pacifi c, and deep (200 -600 m) water fl ows into the Gulf. We propose a biogeochemical method to estimate the vertical component of water exchange assuming a steady state for the concentration of nutrients in the Gulf and using the annual input of dissolved Si needed to support the production of opal accumulated in its sediments. An estimated average opal accumulation of (273.3 ± 6.8, mean ± SE)×10 9 mol SiO 2 year -1 for the whole Gulf was extracted from literature sources. Annual averages for H 4 SiO 4 , at the mouth of the Gulf and for the depth intervals 0 -200 m and 200 -600 m, were used to calculate the annual average vertical component of water exchange between the Gulf and the Pacifi c needed to balance this opal accumulation, i.e., 0.23 ± 0.02 Sv. This relatively low value is only ∼ 3 % of the whole average water exchange. This vertical component value was used to calculate the net annual nitrate input from the Pacifi c into the Gulf, and with the Redfi eld ratio it was transformed to new phytoplankton production for the whole Gulf [(10.67 ± 3.66)×10 9 kg C year -1 ].