Apolipoprotein B (Apo B) is a component of chylomicrons, low-density lipoproteins (LDL), very low density lipoproteins (VLDL), and intermediate-density lipoproteins (IDL) and is the ligand for the LDL receptor. Thereby, Apo B plays a central role in lipoprotein metabolism and in maintaining the normal homeostasis of serum cholesterol levels. Several Apo B restriction fragment length polymorphisms (XbaI, EcoRI, MspI) have been reported to be associated with variation in lipid levels, obesity and/or coronary artery disease. To date, no data are available on relationship between XbaI Apo B polymorphism and lipid levels in Tunisian population. Here, we report frequencies of the XbaI polymorphism of the Apo B gene and we assess the effect of this polymorphism on lipid and lipoprotein concentrations in Tunisian population. Blood samples from 296 Tunisian individuals (112 women and 184 men, aged 51.4+/-9.6 years), were analysed for total cholesterol, triglycerides, HDL-cholesterol and apolipoproteins A1 and B. In parallel, genotyping by means of polymerase chain reaction (PCR) and restriction fragment length polymorphism (RFLP) was performed. The XbaI polymorphism was associated with differences in plasma cholesterol (p=0.04), triglyceride (p=0.02) and apolipoprotein A1 (p=0.004), individuals with the genotype X1X1 have the lowest mean levels and those with the genotype X2X2 have the highest, with the individuals heterozygous for the polymorphism having intermediate levels. According to sex, the XbaI polymorphism effect was only observed for triglyceride in men. Thus, the results demonstrate an influence of XbaI polymorphism of Apo B gene on serum total-cholesterol, triglycerides and apolipoprotein A1 concentrations among Tunisian population.