The marine Achnanthales (Bacillariophyceae), with a particular focus on Cocconeidaceae, were investigated in a variety of habitats from the South Province of New Caledonia (Melanesia, Pacific Ocean). Species diversity ranged from species-poor and scarce assemblages on mangrove mud flats to highly diverse and abundant assemblages on short turfs and scrapings of aerial pneumatophores of Avicennia marina. Macroalgae (in place or as sea leash), as well as feces of Acanthopleura gemmata (invertebrate grazer, living on intertidal rocky environments), contained few diatoms, with low diversity. The New Caledonia assemblages show some taxa previously found in the tropical Indian Ocean. Several noteworthy taxa, listed as rare from Polynesia, are not uncommon in New Caledonia, such as Vikingea gibbocalyx and Cocconeis pseudodiruptoides. The notion of endemism concerning marine eukaryotes is approached.