Members of Prymnesium belong to the division Haptophyta, class Prymnesiophyceae, order Prymnesialesand family Prymnesiaceae. As most haptophytes, members of the genus Prymnesium are unicellular andplanktonic. The most known of these species is the ichthyotoxic P. parvum, which may form nearlymonospecific dense blooms in coastal and inland waters. This species possesses extraordinary plasticityconcerning life survival strategies, and is specifically addressed in this review.Toxins produced by P. parvum have hemolytic properties, that not only kill fish but also co-existingplankton. These substances are allelopathic (when other algae are killed) and grazer deterrent (whengrazers are killed). Allelopathy enables P. parvum to utilize inorganic nutrients present in the surroundingwater without competition from other algal species; and by eliminating its grazers P. parvum reduces celllosses. The paralized microalgae and/or zooplankton, are therefter ingested by the P. parvum cells, aprocess called phagotrophy. P. parvum is also able of osmotrophy, i.e. utilization of dissolved organicmatter. In this review, the cellular characteristics, life cycles, bloom formation, and factors affectingtoxicity, allelopathy, phagotrophy, and osmotrophy of P. parvum are discussed.