Diatoms constitute the most diverse and productive taxonomic microalgae group of the marine phytoplankton. Nanoplanktonic (and picoplanktonic) forms have been recently studied in various parts of the world, showing high densities and considerable contributions to productivity. This study is based on the analyses of water samples collected and filtered from western coasts of Baja California (BC) and Baja California Sur (BCS), on board B/I “Dr. Jorge Carranza Fraser” (INAPESCA) by September-October, 2020. Water samples (3 liters) were collected and filtered at depths of the subsurface chlorophyll maxima, from three stations in front of BC and other three in front of BCS. Superficial chlorophyll had concentrations less than 0.45 mg m^-3 in all stations sampled. For species identification and counting, light (LM) and scanning electron microscope (SEM) were used. Species smaller than 20 µm (even smaller than 3 µm) were recongnized, and many were very frequent, such as Minidiscus chilensis, M. decoratus, M. trioculatus and Mediolabrus comicus, as well as various Thalassiosira species such as T. binata, T. minuscula, T. oceanica, T. profunda, T. tealata, Shionodiscus oestrupii var. venrikae, and the “pennates” Delphineis minutissima, Fragilariopsis pseudonana, Nanoneis hasleae and Nitzschia leei. Highest density in BC appeared in E26 (7.86x10⁴ cel L^-1) and the lowest in E23 (8.93x10³ cel L^-1), whereas in BCS, the highest was in E41 (1.47x10⁵ cel L^-1) and the lowest in E44 (1.02x10⁴ cel L^-1). According to these results and others about coccolithophorids, we consider that coccolithophorids may replace nanoplanktonic diatoms or viceversa, as they are inversely correlated.