Although iterative development can be uncoupled from morphogenesis in plant organs, the relationship between the cell cycle and developmental events is not well established in embryos. Zygotes of fucoid algae, including Fucus and Pelvetia are particularly well suited for studying the interaction(s) between cell cycle progression and the early morphogenetic events, as the establishment of polarity and its morphogenetic expression, i.e. germination, and the first cell cycle are concomitant. We have previously demonstrated that, in Fucus zygotes, various aspects of cell cycle progression are tightly controlled by cyclin-dependent kinase (CDK)-like proteins, including two PSTAIRE CDK-like proteins, p34 and p32, which are synthesised after fertilisation. We show that specific inhibition of CDK-like proteins, either with purine derivatives such as olomoucine and amino-purvalanol or by microinjection of the CDK inhibitor p21(cip1), prevents germination and cell division. Whereas direct inhibition of DNA replication by aphidicolin did not affect polarised development, olomoucine, which has previously been shown to prevent entry in S phase, and other purine derivatives also inhibited photopolarisation. Early microinjection of a monoclonal anti-PSTAIRE antibody also prevented germination and cell division. Only p34 had affinity for amino-purvalanol, suggesting that among PSTAIRE CDKs, this protein is the main target of purine derivatives. Models to account for the simultaneous control of early cell cycle progression and polarisation are proposed.