ABSTRACT. – Observations on net and water bottle samples obtained from marine lakes situated on Mecherchar Island in the Republic of Palau have revealed that Chaetoceros affinis commonly occurred in the meromictic lakes during 2001-2007, but was absent from the holomictic lakes and lagoons. In Mecherchar Jellyfish Lake, C. affinis accounted for 42-93% of the planktonic diatoms during most of the sampled years, representing up to 3.4 x 10⁵ cells/liter. High relative abundances of C. affinis were also recorded from several other meromictic lakes during one of the sampled years, with an absolute abundance of 5.7 x 10⁵ cells/liter (79%) recorded in L-shaped Lake. These observations are the first record of planktonic diatom assemblages in Palauan meromictic marine lakes. The characteristics of these meromictic marine lakes and hypotheses for the recurrence of C. affinis blooms in Mecherchar Jellyfish Lake are discussed herein.