Malyngamide C (1), a chlorine-containing amide of 7(S)-methoxytetradec-4(E)-enoic acid, was found to be a major constituent of the lipophilic extract of Lyngbya majuscula from Fanning Island. The complete structure, including absolute stereochemistry, was determined by spectral and chemical studies of malyngamide C and several derivatives. Three structurally related compounds, viz., malyngamide C acetate (3), deoxymalyngamide C (4), and dideoxymalyngamide C (5), were shown to be minor constituents of the alga. Three new diterpene lactones, 7α,17β-dihydroxy-15,17-oxidospongian-16-one 7-butyrate (2), 7α,17β-dihydroxy-15,17-oxidospongian-16-one 7-acetate (3), and 7α,17β-dihydroxy-15,17-oxidospongian-16-one (4), isolated from the Caribbean sponge Igernella notabilis are reported. The structure of 2 was determined by X-ray analysis and those of 3 and 4 were established by comparison of their spectral data with that of 2.