Vanadium-dependent peroxidase activity in extracts of Ascophyllum nodosum growing in the intertidal region close to Roscoff/France, and algal vanadium levels, followed approximately similar seasonal variation, as deduced from a study lasting from April 2005 to March 2006. High peroxidase (PO) activity was found in extracts obtained from algae collected in between midwinter to spring [approximately 100-190 U per g dry mass (dm), triiodide assay] with a maximum in April. Periods of reduced PO activity lasted from summer to early winter (approximately 50-90 U per g dm). High vanadium levels (1.5-2.2 mg kg(-1)dm) were found in algae collected from midwinter to spring, whereas reduced levels (0.6-1.4 mg kg(-1)dm) were found in summer to early winter.