Three marine planktonic species of the diatom genus Chaetoceros were studied by light and electron microscopy, from net samples collected from different tropical and subtropical areas: coasts of Baja California, the Gulf of Tehuantepec (both in the Pacific Ocean), the Gulf of Mexico (the Atlantic Ocean) and the Indian Ocean. Two of the species, Chaetoceros pseudodichaeta and C. pseudoaurivilli, were originally described by Ikari in 1926 from Japanese waters. This is the second record of them. C. pseudodichaeta is superficially similar to C. dichaeta but detailed morphology differs, especially that of the intercalary setae, which are four-sided in cross-section, with long spines at the edges, and a pattern of two striae between two costae. Chaetoceros pseudoaurivilli is a distinctive species with a characteristic dome- shaped protuberance on terminal valves and the presence of various rimoportulae on, at least, terminal valves of the chain. Finally, C. pseudosymmetricus is a species only reported twice in the Indian Ocean. Its distinctive character is the heteropolarity of terminal setae: one is coarse, long and strongly curved, and the other is delicate, shorter and smoothly curved. It also has a single rimoportula on terminal valves only. The three species are regarded as rare, and consequently our knowledge of their distribution is rather poor. Additional taxonomic comments are provided for the three species.