Specimens of Dictyota pfaffii from Atol das Rocas, Northeast Brazil, afforded the rare dolabellane diterpene 10,18-diacetoxy-8-hydroxy-2, 6-dolabelladiene (1) and the new 10-acetoxy-8,18-dihydroxy-2,6-dolabelladiene (2). Reduction of 1 yielded 8,10,18-trihydroxy-2,6-dolabelladiene (3), also present in the crude extract of D. pfaffii. All three structures were assigned by 1D and 2D NMR spectral data. These substances showed strong anti-HSV-1 activity in vitro but only 3 inhibited the reverse transcriptase enzyme of HIV-1.