biologic0 de1 C. N. R. Via Toiano 2, Arco Felice -Napoli, Italy (Rooaivmd in UK 4 Fettrcuy 1975; amopted for pn?Uoation 19 Much 1975) Numerous fursnoid terpenes have been reported recently from sponges': these have linear C25, C3c and C3s chains or truncated (Cal or Cal) chains. In the main these compounds have been isolated from the family Spongidae (order Dictioceratida). Fursnoeesquiterpenoids (Cls) have also been found in a marine sponge Pleraplysilla spinifera (family Aplysillidae, order Dictioceratida): two are linear end one is mono-cyclic with a new type of sesquiterpene skeleton2. In the course of our program on constituents of sponges we examined the extract of Dieidea callescene (femily Aplysillidae) which showed to be a rich sour ce of furancigseequiterpenee. Acetone extraction of the sponge followed by etherwater partition of the residue and silica gel column chromatography (light-petrg leum) of the ether-soluble fraction' furnished an oil which was separable by column chromatography on 30s AgIW3-Si02 (light-petroleum end increaeing smounta of benzene) into ten new seequiterpenes. These include three of a mono-cyclofax nesane type, nsmed in order of elution pallescensin -1 (I), -2 (2) end -3 (1); and Beven, closely related, having a 2,3_disubstituted fursn ring end two more cycles in their structures, named pallescensina A -G. Iack of material and the unstability of moat of them,prevented extensive chemical investigation and the structure aesignementa are mainly based on spectral grounde,biogenetic considerations and interrelation between them. The spectral arguments suggesting the structures 1 -2 for palleecenein -1, -2 and -3, respectively, are the subject of this report. The following accompai ning papers concern with pallesceneine A -G. All compounds were oils homogeneous on SiOc-AgNOx t.1.c. end g.1.c. (1% OV-1