The aim of this study was to link the variability of the milk fatty acid (FA) composition of tank milk to the production conditions of different farms in France. The milk samples were collected from 10 tours located in lowland using either corn (CL, n=5) or grass (GL, n=5) based diets and 10 tours located in mountain using either corn (CM, n=5) or grass (GM, n=5) based diets. Tank milk samples were collected 5 periods across the year for analysis of milk FA composition. In order to characterize the production system on the day of sampling, surveys were carried out with each farmer involved in the experiment. The period had a significant effect on milk FA composition. The saturated FA (SFA) content decreased during spring whereas monounsaturated FA (MUFA) and 18:3n-3 content increased. Total trans 18:1 content increased during spring and more markedly in G systems. The SFA content was higher for C than for G systems.