Two classes of aliphatic alkaloids, pseudodistomins and penaresidins, were isolated from an Okinawan marine tunicate Pseudodistoma kanoko and a sponge Penares sp., respectively. They have piperidine and azetidine skeletons, respectively; these cyclic amino alcohols are likely to be biogenetically related to sphingosine derivatives. Other sphingosine-related metabolites from marine microorganisms are also described.