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The diatomological assemblages of a sedimentary succession of the deltaic plain of the Colorado River (Argentina) were studied in order to reconstruct the paleoambiental conditions of the delta in response to Holocene sea-level changes. The core was extracted by vibracoring from Las Isletas (LI), located at 22 km from the current coastaline (39 ° 43´5.7 "S, 62 ° 20´49.7" W) and it presented 191 cm of depth. Micropaleontological analysis of 35 subsamples defined three diatom zones (DZ) by a cluster análisis -CONISS-. The sedimentary succession was dominated by small fragilarioids, being Punctastriata glubokoensis the most abundant ; the accompanying flora presented fresh- brackish species and allocton marine-coastal taxa. The base subzone DZIA (191-163 cm, 7,315±64 yr 14C BP) included the sandy lithological units and presented abundant marine-coastal species. The overlying subzone DZIB (163-96 cm) presented little abundance of allocton marine species. The uppermost zone DZII (96 cm from the top) integrated the silt-sandy lithological units and it was formed ca. 2,218±29 yr 14C BP. DZII lacked marine species. The LI sequence represents the infilling of a river-estuarine channel with a tidal influence ca. 7,300 yr 14C BP, that gradually evolved into a fresh-brackish river channel, similar to the current river ca. 2,200 yr 14C BP. During the middle Holocene transgression, greater marine influence would have transported coastal marine species upstream to the LI site; the increased fluvial influence associated with the progradation of the delta lobe would have favored the ubiquity and persistence of the small fragilarioids in the fluvial-delta assemblages of the Colorado River during the last ca. 7,000 yr 14C BP.