Silver salt spectroscopy of seaweeds samples collected from the Qingdao coast from March to August, 1996 showed that mature Laminaria japonica contains about 10.0 μg/g fresh weight of total arsenic; that arsenic content varies greatly within the same frond; that at the center tending to be higher than that at the marginal part. At the frond base, arsenic contents are about three times those at the top. This distribution characteristic relates to the plant metabolism. The more active metabolism some parts of seaweed have, the more arsenic they accumulate.Arsenic contents in Sargassum thunbergii from the Qingdao coast showed clear temporal variations, lowest at the initial stage, highest at maturity. The inorganic percentage in total arsenic showed similar variations. It proved that arsenic contents in seaweed were closely related to its growth cycle. Silver salt spectroscopy is a simple and accurate method of arsenic analysis. On account of its high sensitivity and recovery, it can be regarded as an effective method for determining arsenic in seaweeds.