Nine diterpenes have been isolated from the marine sponge Chelonaplysilla sp. collected in Pohnpei, Federated States of Micronesia. These include the known diterpenes 1-bromo-8-ketoambliol A acetate, dendrillolide A, dendrillolide D, norrisolide, 12-desacetoxyshahamin C, and aplyviolene. Three novel rearranged spongian diterpenes, chelonaplysins A, B, and C, were identified by interpretation of spectral data and chemical correlation with known compounds. Aplyviolene, chelonaplysin B, and chelonaplysin C exhibited antimicrobial activity against the bacterium Bacillus subtilis. The secondary metabolite composition of this collection of Chelonaplysilla sp. is compared to the composition previously reported for samples from Palau.