Zygotes of the Fucales (Phaeophyceae) have been widely used as a model system to study plant cell polarity. Zygotes in the early stages of development are apolar. During the first cell cycle, a gradual cytoplasmic re-organisation, oriented by numerous environmental stimuli, replaces the initial spherical symmetry by a highly polarised morphology. Gradients of cytoplasmic free Ca²⁺ and of the Ca²⁺ binding protein, calmodulin, have been observed in the growing tips of polarised plant cells such as root hairs and pollen tubes. Moreover, calmodulin is essential for budding in Saccharomyces cerevisiae, and localises to the future bud site where it is thought to guide microfilament concentration. Both Ca²⁺ and microfilament localisations have been linked to polarity in Fucoid zygotes, but data regarding the role of calmodulin has been lacking. Fucus serratus zygotes were treated with various inhibitors of cytoplasmic Ca²⁺ mobilisation and of calmodulin activity from 10 to 13 hours after fertilisation, when zygotes are polarised by light. Following treatment, photopolarisation decreased in a dose dependent manner, indicating the requirement for both cytoplasmic Ca²⁺ and active calmodulin in this process. It was hypothesised that localised calmodulin activity was part of the signal transduction pathway between high stimulus and cytoplasmic polarisation. To test this hypothesis, artificially increasing the calmodulin concentration in polarising zygotes to disrupt any endogenous calmodulin gradient was proposed. However, given the evolutionary divergence of the Phaeophyceae, further information regarding the sequence of brown algal calmodulin was required.