The gonyaulacoid dinoflagellate Alexandrium ostenfeldii (Paulsen) Balechet Tangen has recently been identified as the source of spirolides,pharmacologically active macrocyclic imines in natural planktonpopulations and shellfish. The effects of physiological status onspirolide production were studied by comparing nutrient-replete growthof a toxic strain of A. ostenfeldii (SH98) isolated from Nova Scotiawith that of a non-toxic clone (BAHME 146) after dark-induced cellsynchronization. Clones were grown in triplicate 15 L batch cultures at15?1 °C under a 14:10 L/D photocycle at an ambient photon flux densityof 260 µmol m-2 s-1. After 106 h dark adaptation, samples were taken at2 h intervals through three L/D cycles for measurement of chlorophyll a(extracted and in vivo), cell number, cell size, cellular DNA, andspirolide concentration. Although complete cell synchronization was notachieved, cell size variation was related to the L/D photocycle, withsize increasing in the light and decreasing in the dark. Mean cellnumbers decreased during the dark, but net growth was positive duringthe experiment (µ=0.18 div d-1). The extracted chlorophyll a exhibitedthe same trend as the cell concentration, with no apparent shift in theamount of chlorophyll a per cell in relation to the L/D phase. Analysisby liquid chromatography-mass spectrometry (LC-MS) showed that thespirolide profile did not vary significantly, consisting primarily ofthe des-methyl-C derivative (>90% molar), with analogues C, C3, D, D3and des-methyl-D as minor constituents. For the toxigenic clone, thetotal spirolide concentration per unit culture volume was directlyrelated to the concentration of cells and chlorophyll a, but there was adramatic increase in cell quota of spirolides at the beginning of thedark phase and a corresponding decrease in the light. Biosynthesis ofthese polyketide-derived metabolites is apparently affected bylight-dependent events in the cell cycle.