Nodularin (Nod), produced by the brackish/marine cyanobacterium Nodularia spumigena, is a potent hepatotoxin, tumor promoter and is possibly carcinogenic to mammals. It is structurally and toxicologically related to the microcystins, produced by Microcystis aeruginosa in fresh water. A better understanding of the kinetics of Nod production might provide an insight into the physiological and ecological function of cyanobacterial hepatotoxins. The present study presents a simple model simulating the concentration of Nod in N. spumigena KAC66 during phosphorus-limited growth. The main assumption of the model is that the Nod production rate is proportional to the chlorophyll-a (Chla) concentration. The model was tuned to data from phosphorus-limited batch cultures of N. spumigena KAC66 at saturating light and was able to predict 96% or more of the variation in both Chla and Nod concentration. No significant effect of available nitrogen source was found on the Chla-specific Nod production rate although specific growth rates were higher in ammonium and nitrate grown cultures compared to cultures grown with N2 as the sole nitrogen source. Literature data on microcystin production by M. aeruginosa in phosphorus-limited chemostats fitted the model predictions well, except at very low dilution rates (0.1 day−1). The good fit with the proposed model to our own and literature data suggests that the production of hepatotoxic cyanotoxins is not regulated upon growth reduction due to phosphate limitation.