Although comparatively little research has been undertaken into bryozoan secondary metabolites as compared with those of other marine invertebrates, bryozoans have proven to be an excellent source of novel and/or biologically active compounds, with the majority of isolated metabolites being alkaloids. In a continuing survey of the chemical composition of New Zealand marine bryozoans, an extract of Euthyroides episcopalis was investigated. Three novel alkaloids, euthyroideones A-C (1-3), were isolated via MeOH/CH₂Cl₂ extraction, reversed-phase flash column chromatography, gel permeation chromatography, and HPLC. The structures were elucidated using HREIMS, NMR (HMQC, HMBC), and X-ray crystallography (for euthyroideone A), revealing a pyrido[4,3-h]-1,4-benzothiazine skeleton for euthyroideone A, a double bond between C2 and C3 for euthyroideone B, and a 7,8-dehydro structure for euthyroideone C. Biological assays (P388 murine leukemia, antiviral/cytotoxicity) showed no significant activity for most compounds, except euthyroideone B, which exhibited weak cytotoxicity to BSC-1 cells.