The red ascidian Botryllus leachi contains two novel pyrazine alkaloids, botryllazine A (1) and botryllazine B (2) together with the new imidazole alkaloid 2-(p-hydroxybenzoyl)-4-(p-hydroxyphenyl)imidazole (3). The structures of compounds 1-3 were elucidated by interpretation of spectral data, with special emphasis on the analyses of H-1-C-13 couplings. Botryllazine A (1) represents the first example of a marine alkaloid containing a pyrazine nucleus derived from three tyrosine precursors. It is proposed that the biogenetic pathway leading from two tyrosine precursors to botryllazine B (2) involves amide formation and, subsequently, cyclization via imine formation. The cytotoxicity assay results of the new compounds 1-3 against four tumor cell lines are presented. (C) 1999 Elsevier Science Ltd. All rights reserved.