ABSTRACT. – Net phytoplankton samples from the tropical coasts of the Mexican Pacific Ocean, from the States of Sinaloa to Oaxaca, were analyzed to study marine planktonic diatoms of the order Rhizosoleniales, genera Dactyliosolen, Guinardia, Neocalyptrella, Proboscia, Pseudosolenia, and Rhizosolenia. Twenty taxa (18 species and 2 varieties) were recognized and studied by LM, SEM and TEM, in most cases. Three new records for the Mexican marine waters, particularly the Mexican Pacific Ocean, are annotated in this paper: Dactyliosolen blavyanus, Rhizosolenia decipiens and Rhizosolenia fallax. There have been no further records of both Rhizosolenia decipiens and R. fallax, since their original description. Additionally, the species Rhizosolenia clevei var. clevei is studied and illustrated for the first time in the Mexican Pacific. Morphologic details of many species, including complete cells with chloroplasts and the areolae vela of valves and copulae, are provided here. The Rhizosolenia flora in the Mexican Pacific consists of 23 taxa annotated.