Previous work has demonstrated that actin plays important roles in axis establishment and polar growth in fucoid zygotes. Distinct actin arrays are associated with fertilization, polarization, growth, and division, and agents that depolymerize actin filaments (cytochalasins, latrunculin B) perturb these stages of the first cell cycle. Rearrangements of actin arrays could be accomplished by transport of intact filaments and/or by actin dynamics involving depolymerization of the old array and polymerization of a new array. To investigate the requirement for dynamic actin during early development, we utilized the actin-stabilizing agent jasplakinolide. Immunofluorescence of actin arrays showed that treatment with 1-10 microM jasplakinolide stabilized existing arrays and induced polymerization of new filaments. In young zygotes, a cortical actin patch at the rhizoid pole was stabilized, and in some cells supernumerary patches were formed. In older zygotes that had initiated tip growth, massive filament assembly occurred in the rhizoid apex, and to a lesser degree in the perinuclear region. Treatment disrupted polarity establishment, polar secretion, tip growth, spindle alignment, and cytokinesis but did not affect the maintenance of an established axis, mitosis, or cell cycle progression. This study suggests that dynamic actin is required for polarization, growth, and division. Rearrangements in actin structures during the first cell cycle are likely mediated by actin depolymerization within old arrays and polymerization of new arrays.