Two new five-membered-ring peroxide acids, plakinic acid F (3) and epiplakinic acid F (4), and a new peroxide-lactone, plakortolide F (5), were isolated from a sponge of the genus Plakinastrella collected from Felicite Island, Seychelles. The structures were elucidated through spectral analysis. The free acids 3 and 4 exhibit moderate antifungal activity against Candida albicans with minimum inhibitory concentrations of 25 micrograms/mL (SDB) and 3.1 micrograms/mL (RPMI) for 3, and 25 micrograms/mL (SDB) and 6.25 micrograms/mL (RPMI) for 4, respectively. Both also showed moderate in vitro inhibition of Aspergillus fumigatus with IC90's of 25 micrograms/mL.