The lithistid sponge Aciculites orientalis contains three cyclic peptides, aciculitins A-C (1-3), that are identical except for homologous lipid residues. The structure of the major peptide, aciculitin B (2), was elucidated by interpretation of spectroscopic data. The aciculitins consist of a bicyclic peptide that contains an unusual histidino-tyrosine bridge. Attached to the bicyclic peptide are C-13-C-15 2,3-dihydroxy-4,6-dienoic acids bearing D-lyxose at the 3-position. The structures of aciculitamides A (4) and B (5), which are artifacts obtained earlier from this sponge, are also presented. The aciculitins 1--3 inhibited the growth of Candida albicans and were cytotoxic toward the HCT-116 cell line.