Bifurcaria bifurcata, a temperate brown macroalga, is known to show spatial fluctuations in its diterpene content along the northwestern coasts of France (Brittany). In the aim to identify environmental factors which could influence the occurrence of a particular chemical type, several populations of B. bifurcata were collected in summer 2009 and winter 2010. Their chemical composition was studied through thin layer chromatography (TLC) and nuclear magnetic resonance (NMR) analyses. Results showed that specific diterpenes are biosynthesized depending on seasons and abiotic factors, such as hydrodynamism or substratum. Exposed sites were characterized by thalli of B. bifurcata producing two main diterpenoids, bifurcane, and eleganediol, whereas thalli from sheltered sites showed crude extracts containing a major diterpene, eleganolone. On these last sites, another diterpene (bifurcanone) is only expressed in winter and was thus considered as a seasonal chemomarker. The term "chemotype" applied to a population is proposed and discussed.