Recently, increasing attention has been paid to tunicates, since their metabolites frequently possess interesting pharmacological activities.2 In our continuing search for bioactive compounds from Okinawan tunicates,3 we have encountered the tunicate Eudistoma cf. rigida, extracts of which exhibited strong antileukemic activity. In this paper we describe the isolation and structure elucidation of two novel 24-membered macrolides, designated iejimalides A (1) and B (2), with potent cytotoxicity against L1210 (IC50 = 62 and 32 ng/mL) and L5178Y (IC50 = 22 and 1.0 ng/mL) murine leukemia cells in vitro. This is the first isolation of macrocyclic lactones from a tunicate.