Fractions of the acetone extract and a mixture of two diterpenes from the marine brown alga Dictyota menstrualis were prepared with the aim of identifying potential effects on the germination of seeds and on elongation of the radicle and hypocotyl of the weeds Mimosa pudica and Senna obtusifolia. The bioassay on seed germination was performed in controlled conditions of 25 degreeC temperature and a 12 hour photoperiod, while the one concerning radicle and hypocotyl elongation was performed at the same temperature, though adopting a photoperiod of 24 hours. The results varied according to the receptor species, the fraction utilized, and its concentration. TLC analysis of the fractions and comparison with isolated products indicated that the diterpenes pachydictyol A and isopachydictyol A were the most abundant compounds in fraction HE, whereas the diterpene 6-hydroxy-dichotomano-2, 13-diene-16, 17-dial (3) was the most abundant compound in fractions DC and EA. Analysis of less polar fractions (in n-hexane, dichloromethane and ethyl acetate) revealed values of less than 30% inhibition. On the other hand, the ethanol/water fraction was the most active in all instances. The biological activity of these fractions must be due to the presence of known diterpenes and/or sulfated polysaccharides isolated in earlier studies.