The NMR-directed isolation and structure elucidation of nine new nitrogenous hamigeran diterpenoids from the New Zealand marine sponge Hamigera tarangaensis are described. Featured in this set are the oxazole-containing hamigeran M (4) and eight compounds (5a-6a and 7a-8c) related to the constitutional structure of hamigeran D (1). Moderate cytotoxicity in the low-micromolar range against the HL-60 promyeloid leukemic cell line is reported for seven of the new compounds. The structural nature of these compounds suggests that their adducts are derived from an amino acid source and has allowed for revision of the configuration about C-18 of the archetypal compound, hamigeran D, from 1a to 1b. The existence of three constitutionally identical forms of hamigeran Q (8a-8c) requires the involvement of an allo-isoleucine stereoisomer and suggests the intriguing possibility of partial prokaryotic biogenesis of these unusual secondary metabolites.