Contents Summary 295 I. Introduction 296 II. Unicellular, pseudocolonial, colonial and multicellular phytoplankton: definitions, taxonomy and morphology 296 III. Symbioses 298 IV. Physical constraints on size, morphology and motility 298 V. Elemental stoichiometry 302 VI. Allometry of specific growth rates and specific metabolic rates 302 VII. Trophic interactions 303 VIII. Global significance of large unicells, colonies and multicellular organisms 303 IX. Significance of colonies and multicellular organisms relative to large unicells in the phytoplankton 304 Acknowledgements 306 References 306