Abstract Using an ultrasensitive extracellular vibrating electrode, I have studied the membrane-generated electrical currents around the egg of the brown alga, Pelvetia, between fertilization and germination. During this period, the egg chooses an elongation axis and moves wall-precursor vesicles to the prospective growth region where they are secreted. This results in visible oöplasmic segregation which appears under the light microscope as a 1- to 2-μm-thick clear band at the cortex of the growth region. A steady electrical current enters a small region of the membrane and leaves the remainder of the egg's surface as early as 30 min after fertilization. This early spatial current pattern is unstable and shifts position, often with more than one inward current region. However, current enters mainly on the side where germination will occur and is usually largest at the prospective cortical clearing region. The average measured early current density is 0.06 μA/cm 2 at 50 μm from the egg's surface, implying a surface current density of between 0.2 and 1 μA/cm 2 due to the extrapolation uncertainty. At germination the current increases about twofold, resulting in a total transcellular current on the order of 100 pA. Unilateral growth-orienting light reversal stimulates inward current on the new dark side, and subsequent morphological polarity reversal is preceded by electrical polarity reversal. The steady current tends to increase when the external Ca 2+ concentration is increased or the external Na + concentration is decreased, suggesting that the current is carried in part by Ca 2+. This current will generate a transcellular electrical field which may be the force driving the observed oöplasmic segregation.