In zones of high productivity and/or in shallow waters (frontal or upwellings areas, bays and estuaries) the hydromedusae can reach exceptional abundances, denominated 'blooms'. Occasionally, these exceptional abundances are originated from some few individuals that constitute a 'seed population', which is retained in particular places while the environmental conditions are adverse. Liriope tetraphylla is a holoplanktonic and oceanic hydromedusae that often is found forming dense aggregations, which are developed mainly in the Buenos Aires coast during the summer in Argentinian waters (SW Atlantic). The temporal-space distribution and the abundance of L. Tetraphylla in the Subantarctic region were analyzed, with 1458 samples embracing the Argentinian and Uruguayan platform between the 34° and 55°S. In the Buenos Aires coast in particular, the presence or absence of L. Tetraphylla was analyzed into and outside of the Río de la Plata estuary (Argentina-Uruguay). The rhythm of the reproductive periods was also analyzed and the maximum umbrellar diameter of the individuals and the absence or presence of gonad was measured. The presence of a seed population retained in Samborombón Bay, inside the frontal area of the Río de la Plata estuary is discussed. The distribution area of L. Tetraphylla in the Subantarctic region embraced shelf, coastal and estuarial waters until the 40°S. Most of the registrations corresponded to the Río de la Plata estuary, with smaller salinities than 30 units of salinity (us) and smaller depths than 50 meters. The biggest density was registered to the SE of Samborombón Bay with 381,9 ind/m3. In Samborombón Bay (inside estuary) the species was registered the whole year, with a maximum of abundance in summer and the minimum in spring, while that outside the estuary the species was reported only in the period February-May with significantly smaller values, being absent the rest of the year. The maximum size umbrellar of the individuals in Samborombón Bay was registered in spring and a period of recruitment was observed during December-February. Individuals with gonads were found the whole year, always with a same or bigger diameter to 5 mm. In the SW Atlantic the distribution south of L. Tetraphylla is limited to warm tempered waters (approximately until 40°S), being the values of abundance that it reaches in the Río de la Plata estuary much bigger to the registered in other regions of world. Also, their mainly coastal distribution contrary to their character of oceanic species would be closely related with the high productivity of the estuarial front and with the convergence of marine and estuarial waters that takes place in the estuary and that make that the estuary acts like a retention area. For this reason, the presence the whole year of a population seed in Samborombón Bay is pointed out. The importance of the frontal area of the Río de la Plata estuary as a possible area of accumulation and retention for jellyfish and others planktonic organisms, is remarked. Tesis (Licenciatura). Realizada en el INIDEP (Instituto Nacional de Investigación y Desarrrollo Pesquero- Mar del Plata, Argentina) Unpublished distribución geográfica Hidromedusa abundancia variación estacional Hydromedusae Liriope tetraphylla historia de vida