Karenia selliformis strain GM94GAB was isolated in 1994 from the north of Sfax, Gabes gulf, Tunisia. This species, which produces gymnodimine (GYM) a cyclic imine, has since been responsible for chronic contamination of Tunisian clams. A study was made by culturing the microalgae oil enriched Gaillard f/2 medium. The influence of growing conditions oil toxin content was studied, examining the effects of (i) different culture Volumes (0.25 to 40 litre flasks), (ii) two temperature ranges (17-15 degrees C et 20-21 degrees C) and (iii) two salinities (36 and 44). Chemical analyses were made by mass spectrometry coupled with liquid chromatography (LC-MS/MS). Results showed that (i) the highest growth rate (0.34 +/- 0.14 div d(-1)) was obtained at 20 degrees C and a salinity of 36, (ii) GYM content expressed as pg eq GYM cell(-1) increased with Culture time. The neurotoxicity of K. selliformis extracts was confirmed by mouse bioassay. This study allowed us to calculate the minimal lethal dose (MLD) of gymnodimine (GYM) that kills a mouse, as a function of the number of K. selliformis cells extracted.